Sort of like an adventurous circus tamer.
The beastlord is one of the most capable solo classes in the game, even though they only wear leather armor. Their animal companion is a capable tank, and they have many spells that they can use to keep it healed. Wisdom is their most important statistic, and should be boosted to the top at character creation.
They can't cast spells until level 9, but that doesn't take nearly as long nowadays as it once did. They have access to some weaker versions of shaman buffs, adding some group utility to the beastlord other than the damage that they do. They do most of their damage through their beasts, but they can also melee pretty effectively using light weapons. They lack a snare or root spell, so it makes it a bit harder to solo as effectively as classes like the necromancer or the magician.
Their slow spells are similar to those of the shaman, and are excellent for all situations. Make sure that you keep the slow up as much as possible so that you can ensure that your pet stays alive. They also have a special line of mana and health regeneration buffs along with an attack power-increasing spell line called Ferocity. They're a highly versatile class whose role changes depending on how you keep them geared. Their pets can tank well, do damage, or the beastlord can even focus on their ability to heal their pet and the group. Their buffs and debuffs keep them in high demand, even though their lack of a clear speciality does hamper them somewhat.
The type of pet that the beastlord gets depends on the race of the caster. Barbarians get a wolf, Iksar get a scaled wolf, Ogres receive a bear, Trolls get a crocodile and Vah Shir receive a tiger. While they will spend a lot of their early levels in melee, at higher levels they are more focused on spell casting. Still, it helps to keep your strength and agility high, while keeping wisdom as the chief statistical priority. Manage your pet intelligently, and you will do well as a beast lord.
Once you get a hang of things, it becomes easier to fall into a pattern of play that's easy to maintain. They're one of the fastest soloing classes in the game, with little downtime, although their lack of crowd control makes it difficult for them to fight in certain crowded areas.