In short: A Beastmaster has an affinity towards animals and has a form of limited telepathy with them. The Beastmaster can befriend an animal so it will no longer attack and can try to induce them to become a follower. Once the animal follows him, he can communicate mentally with the animal over any distance and may see what it sees. He gains animal lore with respect to the animal and when the Beastmaster goes up in level, all of his animal followers gain an additional hit point. At 9th level, the Beastmaster can summon an animal horde (which takes one week).
He gains +5% to Hide in Shadows. When one of his animals is wounded, the Beastmaster feels the pain and suffers a -2 penalty on all rolls in the next round. If the animal is killed, this penalty lasts for 24 hours. The beastmaster suffers a -1 reaction penalty with NPCs, -2 if they are civilized aristocracy.
Description: A wanderer, the Beastmaster has a natural affinity for animals; in fact, he has a limited form of telepathic communication with them. This is often the result of a magical bond with the animal kingdom, formed either at the time of his birth or upon reaching young adulthood. Unlike other adventurers, the Beastmaster does not command, train, or control his animal companions, rather they are his friends and comrades in arms. Misunderstood and feared by nobles and common folk alike for his unnatural abilities with animals, the Beastmaster seldom stays in one place for long, nor is he comfortable in civilized lands.
Beastmasters tend to walk alone, accompanied only by fierce natural beasts that are their friends and allies. Traveling the fringes if settled lands, the Beastmaster has small use for the trappings of civilization, but even less for the minions of evil, particularly those of a magical or priestly nature. Thus, a Beastmaster will often find himself aiding the oppressed or enslaved, and pitted against the cruelties of evil priests or wizards and their allies.
Weapon Proficiencies: A Beastmaster starts only with weapons he can make himself: axe, club, dagger, dart, javelin, knife, quarterstaff, sling, or spear.
Armor/Equipment: Beastmasters start only with leather armor and weapons they have made.
Stealth: The Beastmaster has a +5% chance to hide in natural surroundings.
Animal Henchmen: Although a Beastmaster has no special followers at high level, he can acquire normal or giant animals as henchmen. He may acquire them at any level, and their number depends on his charisma.
Animal Telepathy: The Beastmaster can establish telepathic communication with any normal or giant animal within 30', if he does nothing else in that round. The animal must have a minimum intelligence of 1. This has the following benefits: The Beastmaster can communicate to the creature that he desires its friendship. If the offer is sincere, the creature can be calmed and will not attack unless it is attacked. The Beastmaster can recruit an animal he has befriended as a henchmen if he is not at his limit and if the creature fails a saving throw vs. rods. The save is penalized by -1 for every 3 level of experience of the Beastmaster.
Animal Bonding: The Beastmaster forms a mental bond with any animal he recruits as a henchman. There is no distance limit, but this ability does not cross planar boundaries. This bond has the following effects: He can see though the eyes of the animal by concentrating on the mental link. Each time the Beastmaster gains a level, all of his current henchmen gain an additional hit point.
Animal Horde: At 9th Level, the Beastmaster can summon a horde of wild animals to fight for him. They must come from a land he controls, and it takes one week to gather them. They can only be brought together for some great purpose that can be explained simply. Up to 100 hit dice of animals per level of the Beastmaster will come. The horde will stay together for 1 week per level of the Beastmaster. There is no record of a Beastmaster summoning a horde more then once per year.
Empathetic Shock: The Beastmaster feels great pain when one of his henchmen is wounded, suffering a -2 to all hit rolls in the next round. If he is linked with the animal when it is killed, he suffers a -2 to hit for the next 24 hours.
Unruly Allies: The Beastmaster's animal henchmen are free to come, go, or act as they will. Any attempt to restrict or regulate their freedom, or ignoring their needs and desires, will result in resentment, sulkiness, and possible abandonment.
Outcast: The Beastmaster suffers a -1 to his reaction roll by common NPCs, and a -2 when dealing with a civilized aristocracy. Further, his maximum effective Charisma when dealing with his own race is a 15.
No Fortress: At no time will the Beastmaster build a fortress, though at 9th level he may establish himself as the protector of an area of land equal to a barrony.
Source: The Complete Ranger's Handbook