Tower Defense games are strategic games, where you place towers along a path to stop an ambush.
These Towers have various effects, from simple damage to giving slownes or in some games even can teleport.
The Towers can be upgraded to increase their effect.
Both, building and upgrading the towers, costs some kind of money. You earn it by killing the ambushers.
Usually one game is divided into different waves, where every wave of an attack has stronger or more ambushers than the one before.
The goal of the game is to survive as long as possible, in some rare cases, you play some kind of story with different stages, where every stage has a defined number of waves.