There are 11 different types of units in Lords & Knights:
Follow the above links to learn more about each of these units.
Spearmen, Crossbowmen and Armoured horsemen are generally regarded as defensive units. These are units which are good defensively but not so good when attacking. Principly use these units to protect your castle or castles.
Swordsmen, Archers and Lancer horsemen are generally regarded as offensive units. These units are better at attacking than defending.
Hardcarts and Oxcarts have no offensive or defensive ability. They are used solely for moving wood, stone and ore from one castle to another. Oxcarts are slower than handcarts but can transport 5 times as much. All the other units can also be used to transport resources, but individually they cannot carry anywhere near as much as even a handcart.