Guild Wars: Eye of the North is going to be the first actual "expansion" to Guild Wars, a game that is still enjoying steadily growing popularity, and is slated for release on August 31st. After the release of Guild Wars: Eye of the North, the development team will be switching immmediately to focusing on Guild Wars 2.
Since there is already an installed base of players, NC Soft is focusing on supporting these existing players; the new expansion focuses on high level chracters, without including low level "starter" areas. This allows development for current players who are looking for new challenges. Guild Wars: Eye of the North is intended as a sequel and successor to the original Guild Wars gameplay, and returns the players to Tyria, but this time to explore it in more depth. Specifically, this expansion focuses on the dungeon crawl experience, with the puzzles and traps you'd expect to find in a dungeon crawl. Guild Wars: Eye of the North includes 18 multi-level dungeons to explore.
Another way that NC Soft is supporting it's existing GW players is by providing a meaningful migration path to Guild Wars 2. When Guild Wars 2 comes out, you won't carry over your items directly, but you will get one item for each item that you had in Guild Wars, and all of these items will only be obtainable via this migration process.