Guild Wars Factions is the latest campaign in the Guild Wars series of games from NCSoft.
In case you've somehow escaped Guild Wars, it is a free to play online RPG. You have to buy the boxed game, but once you own it, you can play online as long as you like. The game is MMO-style in that you can meet up with lots of other players in central town areas, but the missions themselves take place in instanced areas that only you and the rest of your party can enter.
Guild Wars Factions is a stand-alone game, however if you own Guild Wars as well, then you can take your Factions characters into Guild Wars areas, and vice-versa.
Guild Wars Factions brings a number of changes and improvements, as well as extra content for those who like the questing and role-play side of things.
For starters, the graphics have been improved, and there is an entire new area to explore. In terms of PVE content, new players will find that the experience gain on the tutorial island is incredibly fast. The tutorial series of quests is very in-depth, and most people will be in the level 13+ range by the time they finish the quests.
Earning gold is also faster in the Factions campaigns, allowing even small guilds to quickly purchase their guild cloak and other goodies.
Guild Wars Factions adds a lot of new skills to the game (300, apparently, but I haven't counted), both
normal and elite ones. It also adds two new classes - Ritualists and Assassins. Ritualists in particular are a very interesting and versatile profession, that can be played in a number of ways. They have summoning skills which last a short time, and on death have various effects. Assasins are a lightly-armoured, high damage profession that live up to their name and have some very flashy skills.

The majority of Guild Wars really focusses on PVP - there are 20 levels in the game, but getting through them is very fast compared to most MMO style games. There is even the opportunity to create a special PVP only character, that can not take part in quests or play the 'normal' game, but can take part in certain PVP events.
There is a new PVP area in Factions, and also some new options, scaling up from small scale battles to huge alliance battles. There are also PVE-quests that allow up to 16 people to take part at once. The guild management interface has been improved also.
The first thing that struck me about Factions is the improved monster AI - in Guild Wars, if you ever needed to walk back through an area (perhaps you were finishing off an old quest) the monsters would aggro on you and your party, even if they were far, far lower level than you. This was very irritating, and wasted a lot of time. Now, if a monster is far enough below your level that you would not gain experience for killing it, they seem to ignore you for the most part. Its a small change, but important - it makes me feel like I'm not being artificially slowed down when playing the game.
Another change from standard Guild Wars is the terrain. Anyone who has ever played Star Wars Galaxies will know that the game was completely open terrain wise, then they introduced the Wookiee planet Kashyyyk, which was a maze-like, walk only where we tell you environment. Standard Guild Wars felt a lot like Kashyyyk - a small bump in the terrain may as well have been a mountain that would not let you past. In Factions, the terrain is a lot more open and you can walk around more freely.

I did feel slightly cheated by the promise of being able to take my normal characters to the new island of Cantha, and my Factions characters to Tyria. Yes, it is possible, but I had hoped to be able to create a newbie and teleport straight over to level with a friend. In truth, your Factions characters must complete the tutorial island series of quests, and your standard Guild Wars characters have to reach Lions Arch. Granted, that stuff doesn't take long, but if you were hoping to try out a new character, say, a Ritualist in the original game with a friend that doesn't have Factions yet, then you're in for a let down if you wanted to level up together. The Factions character is likely to end up being a lot, lot larger than the regular character in a very short period of play.
That gripe aside, I enjoyed my new characters immensely. The storyline is strong, and you are carried from mission to mission in a nice way - the side quests feel like they have some purpose, and aren't just 'Get this'/'Kill this'/'Go here'. Some of the quests differ slightly for each profession, especially in the early stages of the game. An Assassin might have to kill someone, where a Monk would have to heal an NPC while they were fighting, for example.
If you liked Guild Wars, then you will enjoy the new content of Factions, and the improvements it makes. If you did not enjoy Guild Wars, then this may not be the game for you. Guild Wars is not a full-scale MMO, and if you are looking for that kind of depth and complexity then you may as well look at a different game instead. Guild Wars / Factions are both ideal for people who want something similar to an MMORPG, but on a smaller time scale. For the 'two hours a night after work' player, the combined campaigns of Guild Wars and Guild Wars Factions could keep them occupied for a long time, not to mention that NCSoft are very good about frequently releasing extra content and new campaigns.
I would recommend Guild Wars Factions to anyone who is looking for an online RPG. Those after something to replace a full scale MMO may find it lacking, but if you look at it from the perspective of a regular multi-player RPG then you will likely enjoy it, and find the regular updates a breath of fresh air.
Considering the price (just buy the box, then you can play for life) can you really go wrong?