Unvanquished is a free first person shooter (FPS) game, based around a World War Two scenario. It's open source software, meaning that it's free to download and use. The objective of each side is to attack the opposing base from a position of strategic advantage. By using the keyboard or mouse, the player is able to make quick and accurate movements throughout the battlefield to turn the tide of war in your favor.
If you're familiar with military games you'll be aware that most are designed to give the player a feeling of adventure. Although, there are quite a few games out there that take this to an extreme by putting the player into alien habitats or spaceships where they have to survive, fight off fierce battles with huge bosses, and generally just survive the first level. However, in Unvanquished you will find that the action takes place primarily on the keyboard or the gamepad, which often leads to a more immersive gaming experience. As such, even if you're not accustomed to playing first-person shooters, you will find that this is an extremely engrossing and thrilling game with strong strategic elements.
To download and play unvanquished you will need to find an open source application that you can use to host the server (the server is where all your game data is stored), as well as to connect to the multiplayer server (this will allow you to connect with other players). There are a number of free applications available for both these purposes. However, it's strongly recommended that you download Unvanquished as a premium game instead. It's free, it's fast, and it offers a very stable playing experience.