There are primarily five different types of units. These include: heroes, normal units, summoned units, monsters and titans.
Heroes are based upon Classes. There are 7 different Classes: Cleric, Druid, Fighter, Paladin, Ranger, Rogue and Wizard.
You can acquire heroes through: Spells (Arcane: Summon Hero, Summon Champion), Inns (I think a 5% chance per spawn), Random World Events (I think it depends upon what your fame level is, and how many heroes you already own) or Trading with another Sorcerer Lord.
You can only have up to 6 heroes at any time.
Heroes start relatively weak, but with experience and equipment, can become extremely powerful!
These are units that your cities can produce. These units vary depending upon the race of the city, and vary in strength depending upon the city improvements available.
For the purpose of sorting, we will sort according to Race.
These are units you can summon using starting and/or researched spells. Most summoned units cost mana per turn instead of gold and/or food. They often come with an array of special abilities which make them suitable for certain roles or can be used to fill the gaps in your race's military composition.
For the purpose of sorting, we will sort according to Element Circle.
Monsters are found guarding world structures. They can also be spawned from these world structures to become wandering creatures. Certain Monsters can be summoned or bought from Inns.
Titans are extremely powerful units. They can be recruited to your cause at the thinking man statue world structure after you have defeated the guardians. They can also become wandering creatures at certain points in the game.