Battle for Wesnoth is a free, tactical, turn-based game in which you command fantasy armies in a variety of scenarios. The game follows a relatively familiar pattern: Beginning with a few units, you must acquire resources, summon troops, and accomplish some pre-determined goal, such as "Kill the orc chieftain" or "Escort the caravan across the map."
Each unit in Battle for Wesnoth represents an individual, not an army, distinguishing it from similar games, such as most iterations of Heroes of Might and Magic. This distinction matters, because units can gain experience and advance, becoming more powerful and unlocking new abilities. In many other games, this is something only 'leader' or 'hero' units can do. This, in turn, adds an interesting twist to tactics: Whoever gets the killing blow on an enemy unit gets the bulk of the experience points. If you let just anyone strike without paying attention to their current XP, it will be harder to get a unit to level up...but trying to manipulate things to make sure the level-up-likely unit gets to attack can lead to serious blunders. You can "recall" experienced units from prior scenarios in a campaign, so experience accumulated in one scenario is not lost, and scenarios in long campaigns are balanced on the assumption you will be bringing advanced units with you.
Typically, a Battle for Wesnoth campaign begins with one or more leaders (the only characters who can recruit new troops), a small amount of gold, and a straightforward goal explained via a sequence of dialog boxes featuring NPC conversation... sort of a storyboarded cut scene. You recruit troops in a fortress, then start exploring the map. Moving a unit into a village will flag the village and cause it to start producing gold, which you will need to recruit more units. You will find roads, forests, mountains, and, eventually, the enemy. Then the fun begins.
To battle, you command a unit to move adjacent to a foe. Even those with "ranged" weapons stand toe-to-toe. Terrain and time both matter--an elf in a forest has a huge defensive advantage, cavalry is potent on the open road but hindered in hills and swamps, orcs fight better in the dark, and so on. Learning how to strike when you're strong and the enemy is weak is the key to victory, as is learning how to position troops and protect key units. Due to the turn-based nature of the game, you can ponder your moves as long as you wish--Battle for Wesnoth has a menu item which shows likely enemy moves to aid in your planning.
Combat itself consists of a series of strikes and counter-strikes. The ideal situation is an attacker with a ranged weapon against an enemy without one--the enemy cannot strike back! Be warned, though, that once the turn is done that unit can't move until the next turn. If your archer fails to kill the enemy troll, the troll may very well rend it in the next move.
Several linked scenarios form a campaign, and most scenarios contain triggered events which advance the story. For instance, finding a group of besieged soldiers who will be your allies (computer-controlled friendly units) or locating a critical NPC who will unlock a new scenario goal.
There's very little to complain about in Battle for Wesnoth, if you enjoy this style of gameplay in the first place. I encountered no glaring bugs. The starter scenarios I found to be very easy once I'd mastered the basics, but more advanced scenarios proved much more challenging, so don't be put off if your first few games seem too simple. There's an active development and player community.
Since Battle for Wesnoth is an open-source game without any major company behind it, it's possible you may encounter glitches due to specific graphic cards, drivers, or hardware features (I didn't, but I have in other games with the same kind of development model). If this occurs, the online forum is the best place to go for help.
With a price of "free," if you like turn-based tactical games, it's hard to find a reason not to try out Battle for Wesnoth.