Familiars are magically linked to their masters. In some sense, the familiar and the master are practically one being. That's why, for example, the master can cast a personal range spell on a familiar even though he can normally cast such a spell only on himself. A familiar is a normal animal that gains new powers and becomes a magical beast when summoned to service by a sorcerer or wizard. It retains the appearance, Hit Dice, base attack bonus, base save bonuses, skills, and feats of the normal animal it once was, but it is treated as a magical beast instead of an animal for the purpose of any effect that depends on its type. Only a normal, unmodified animal may become a familiar. Thus, a druid/sorcerer can't use her animal companion as a familiar.

A familiar also grants special abilities to its master (a sorcerer or wizard), as given on the table below. These special abilities apply only when the master and familiar are within 1 mile of each other.

Levels of different classes that are entitled to familiars (such as sorcerer and wizard) stack for the purpose of determining any familiar abilities that depend on the master's level.

BatMaster gains a +3 bonus on Listen checks
CatMaster gains a +3 bonus on Move Silently checks
HawkMaster gains a +3 bonus on Spot checks in bright light
LizardMaster gains a +3 bonus on Climb checks
OwlMaster gains a +3 bonus on Spot checks in shadows
RatMaster gains a +2 bonus on Fortitude saves
Raven¹Master gains a +3 bonus on Appraise checks
Snake²Master gains a +3 bonus on Bluff checks
ToadMaster gains +3 hit points
WeaselMaster gains a +2 bonus on Reflex saves
¹A raven familiar can speak one language of its master's choice as a supernatural ability.
²Tiny viper.

Familiar Basics:

Use the basic statistics for a creature of the familiar's kind, as given in the Monster Manual, but make the following changes:
Hit Dice: For the purpose of effects related to number of Hit Dice, use the master's character level or the familiar's normal HD total whichever is higher.
Hit Points: The familiar has one-half the master's total hit points (not including temporary hit points), rounded down, regardless of actual Hit Dice. For example, at 2nd level, Hennet has 9 hit points, his familiar has 4.
Attacks: Use the master's base attack bonus, as calculated from all his classes. Use the familiar's Dexterity or Strength modifier, whichever greater, to get the familiar's melee attack bonus with natural weapons. Damage equals that of a normal creature of the familiar's kind.
Saving Throws: For each saving throw, use either the familiar's base save bonus (Fortitude +2, Reflex +2, Will +0) or the master's (as calculated from all his classes), whichever is better. The familiar uses own ability modifiers to saves, and it doesn't share any of the other bonuses that the master might have on saves (from magic items feats, for example).
Skills: For each skill in which either the master or the familiar has ranks, use either the normal skill ranks for an animal of that type ort master's skill ranks, whichever are better. In either case, the familiar uses its own ability modifiers. Regardless of a familiar's total skill modifiers, some skills (such as Craft) may remain beyond the familiar ability to use.

Familiar Ability Descriptions

All familiars have special abilities (or impart abilities to their masters) depending on the master's combined level in classes that grant familiars, as shown on the table below. The abilities given on the table are cumulative.

Master Class LevelNatural Armor Adj.IntSpecial
1st-2nd+16Alertness, Improved Evasion, Share Spells, Empathic Link
3rd-4th+27Deliver Touch Spells
5th-6th+38Speak with Master
7th-8th+49Speak with Animals of its Kind
11th-12th611Spell Resistance
13th-14th+712Scry on Familiar

Natural Armor Adj.: The number noted here is an improvement to the familiar's existing natural armor bonus. It represents the preternatural toughness of a spellcaster's familiar.
Int: The familiar's Intelligence score. Familiars are as smart as people, though not necessarily as smart as smart people.
Alertness (Ex): The presence of the familiar sharpens its master's senses. While a familiar is within arm's reach, the master gains the Alertness feat.
Improved Evasion (Ex): When subjected to an attack that normally ws a Reflex saving throw for half damage, a familiar takes no damage if it makes a successful saving throw and half damage even if the saving throw fails.
Share Spells: At the master's option, he may have any spell (but not [spell-like ability) he casts on himself also affect his familiar. The familiar must be within 5 feet at the time of casting to receive the benefit. If the spell or effect has a duration other than instantaneous, it stops affecting the familiar if it moves farther than 5 feet away and will not affect the familiar again even if it returns to the master before the duration expires. Additionally, the master may cast a spell with a target "You" on his familiar (as a touch range spell) instead of on himself. A master and his familiar can share spells even if the spells normally not affect creatures of the familiar's type (magical beast).
Empathic Link (Su): The master has an empathic link with his familiar out a distance of up to 1 mile. The master cannot see through the familiar's eyes, but they can communicate empathically. Because of the limited nature of the link, only general emotional content (such as fear, hunger, happiness, curiosity) can be communicated. Note that the low Intelligence low-level master's familiar limits what the creature is able to communicate or understand, and even intelligent familiars see the world differently from humans, so misunderstandings are always possible. Because of this empathic link, the master has the same connection to an item or place that his familiar does. For instance, if his familiar has seen a room, the master can teleport into that room as if he has seen it too.
Deliver Touch Spells (Su): If the master is 3rd level or higher, a familiar can deliver touch spells for him. If the master and the familiar are contact at the time the master casts a touch spell, he can designate his familiar as the "toucher." The familiar can then deliver the touch spell just as the master could. As usual, if the master casts another spell before the touch is delivered, the touch spell dissipates.
Speak with Master (Ex): If the master is 5th level or higher, a familiar and the master can communicate verbally as if they were using a common language. Other creatures do not understand the communication without magical help.
Speak with Animals of Its Kind (Ex): If the master is 7th level or higher, a familiar can communicate with animals of approximately the same kind as itself (including dire varieties): bats with bats, rats with rodents, cats with felines, hawks and owls and ravens with birds, lizards and snakes with reptiles, toads with amphibians, weasels with similar creatures of the family Mustelidae (weasels, minks, polecats, ermines, skunks, wolverines, and badgers). Such communication is limited by the intelligence of the conversing creatures.
Spell Resistance (Ex): If the master is 11th level or higher, a familiar gains spell resistance equal to the master's level + 5. To affect the familiar with a spell, another spellcaster must get a result on a caster level check (1d20 + caster level; see Spell Resistance) that equals or exceeds the familiar's spell resistance.
Scry on Familiar (Sp): If the master is 13th level or higher, he may scry on his familiar (as if casting the scrying spell) once per day.

Alternate Familiars

If a DM wishes to give undead sorcerers and wizards greater scope for their familiars, she may consider using one of the three following alternatives.

Tiny Monstrous Centipede Poisonous bite
Wiggling MaggotMaster adds +2 to turn resistance; master can have maggot become fly¹
Giant Fly²Master gains a +2 bonus on Reflex saves
¹The wiggling maggot's master can have the maggot transform into the giant fly familiar as a full-round action. This transformation can happen only once. The maggot thereafter will be a giant fly.
² see Giant Fly

Familiars Available using the Improved Familiar Feat

FamiliarAlignmentArcane Spellcaster Level
WorgNeutral evil3
Blink dogLawful good5
Hell houndLawful evil5
HowlerChaotic evil7
Winter wolfNeutral evil7
Beholderkin, eyeballNeutral evil5
Cat, TressymNeutral5
ImpLawful Evil7
PseudodragonNeutral good7
QuasitChaotic evil7
Night hunter batNeutral evil5
Formian WorkerLawful neutral7
Shocker lizardNeutral5

Familiars at Epic Level

Special abilities granted to a familiar continue to grow as the level of its master increases beyond 20th, as shown on Epic Familiar Special Abilities (below). Even though the table shows advancement only to 42nd level, that is not the limit. The patterns in the table continue infinitely. Familiar special abilities gained at less than 20th level also continue to improve.

Epic Familiar Special Abilities
Master Class LevelNatural Armor Adj.IntSpecial
21st–22nd+1116Familiar Spell
31st–32nd+1621Familiar Spell
41st–42nd+2126Familiar Spell

Familiar Spell: The familiar gains the benefit of the Familiar Spell epic feat for the spell its master chooses.

Disclaimer: Some of the above information comes from www.d20srd.org

Magic in the Realms