In Battle for Wesnoth you take control of a king and his fortress in an attempt to conquer other kings. To do this you hire various units to do your bidding.
Battle for Wesnoth allows you to choose between several ages, each containing their own set of units.
There are two ages included, one using a Light vs. Dark theme, the other containing various races like Trolls and Nagas.
There are many different units with unique abilities, but most follow a typical Rock-Paper-Scissor schematic. Each unit is usually powerful against another specific unit, but weak against another.
Battle for Wesnoth requires a high level of tactical thinking but it doesn't take long before you'll find a strategy that works for you.
The game can be played in single-player mode through the use of several campaigns, as well as in multiplayer mode which is implemented via Internet or LAN.
Almost everything in the game can me modded. This includes units, levels, scenarios, and campaigns. There is even a map editor included.
All in all, Battle for Wesnoth is a very good game and it deserves to be played.