There are six types of MOB units that can be created, as well as the main Leader units (or Charisma Units, as they are sometimes called).

Gatherer units

Used to collect resources in the form of Gaia Stones and Treant Fruit. They have very little attack or defense power. Have the terrible tendency to go galavanting off into enemy territory trying to find an alternate route to the resources you pointed them to.

Ground Units

Basic attack units for melee combat. Weak against Heavy, strong against Missile.

Heavy Units

Heavy attack units for melee combat. Weak against Flying units, strong against Ground.

Flying Units

Units that melee attack from the air, or at most from a short distance, and are weak to projectiles. Weak against Missile, strong against Heavy.

Missile units

Units that can attack from some distance, but are vulnerable to melee. Weak against Ground, strong against Flying.

Special units

Units that have a special ability (such as stealth or unit spawning). They are generally not weak to any particular kind of attack. Often the more costly units to spawn.

As you can see, there is a closed weakness diamond created by the main battle units which is very handy to keep in mind when you want superiority over a particular group of MOBs. Units that are weak against another kind will only inflict half the damage on that unit type, and receive double from it.

So, for example, an Missile Unit will inflict double damage on an Flying unit, while the Flying unit's attacks will be halved against the Missile Unit, but a Heavy Unit unit will inflict double the damage on the Archer when all its attacks in return are halved. Effectively, you make your units 4 times more powerful when you send them against the right type of MOBs.

The other modifier for the units is the terrain they can handle.
Terrestrial units can only handle basic terrain, and little in the way of steps.
Saltatory units can handle moderate changes in terrain height (1 unit of height).
Aquatic units can travel over water, and do so faster than travelling on land.
Flying units can travel pretty much anywhere they please.