Brute: Brutes excel at high damage in close quarters while having a lot of HP themselves. They have low defenses to compensate and are simply meant to be used to get in the parties face. Brutes love cover and broken lines of sight so that they are not harassed by ranged attacks. A good example of a Brute would have to be an Ogre.
Soldier: Soldiers have high defenses with average HP, and attacks. They serve as the tanks for fellow monsters, absorbing blows, and discouraging attacks against their weaker comrades. Soldiers love narrow funnel points, that channel all of the enemies towards them one by one. A good example of a Soldier would have to be an Iron Golem.
Artillery: Artillery have high damage range spells, but very low HP and defenses. These guys stay on the backline and rain fireballs on the party. Artillery loves the high ground and having cover to duck in and out of while they rain attacks from afar. A good example of Artillery would have to be a Mind Flayer.
Skirmishers: Skirmishers have average stats but above average mobility. They try to weave in and out of combat, and try to aim for the squishies at the back of the party. Skirmishers love open-ended battlefields with lots of cover. This gives them the opportunity to dive into the back line of the party. A good example of a Skirmisher would have to be a Bullete.
Lurker: Lurkers have below average health and defenses, but have an ability that makes them difficult to target. They wait, hiding in the shadows until they can ambush some poor PC. Lurkers love having places to hide to ambush the party from. A good example of a Lurker would have to be a Cloaker.
Controllers: Controllers are the monsters who try to force the PCs into disadvantageous situations. They will move the PC's around, have the party make saving throws, and put the party in a rough spot. Controllers love terrain that has negative aspects to it, such as acid pools so that they can force the party to move towards the dangerous terrain. A good example of a controller would have to be a Beholder.
Leaders: Leaders are special monsters who give bonuses to the entire group just for simply being there. This trait makes them much more of a target but makes your group a far greater threat. Leaders love terrain that allows them to see the whole battlefield while keeping them safe from any immediate attacks. A good example of a leader would have to be a Hobgoblin Warlord.