Evolution is a multiplayer modification for Half Life 2. It features at least 10 new levels, new weapons, 7 game modes, both old and new, and the ability to customize your character's abilities during the matches in order to tune your powers to different situations. -Taken directly from the FAQ.
The game will feature a complete set of new abilities that will grant the player new powers depending on how advanced they are into the new ability categories. These abilities will be able to be changed in real time throughout the matches, as well. These powers will be the cornerstone of the modification, and everything else in the modification will center around these abilities.
To find out more information about Evolution, please visit the main webpage.