We recently reported that Guild Wars 2 was heading to the Steam platform, but surprisingly, it seems ArenaNet has had a change of heart, at least for the time being.
The company has announced that they intend to put the steam release "on hold" while they focus on the development of the End of Dragons expansion, as confirmed in their latest blog post;
Our number one priority right now is crafting Guild Wars 2: End of Dragons with the high quality you expect and deserve. Additionally, we're full speed ahead on delivering an epic climax to The Icebrood Saga that lives up to (and we hope exceeds) previous Living World finales.
It still seems likely that the game will make its way to Steam eventually, given other titles from ArenaNet such as RuneScape have already made the trip to Valve's platform.
When it does, if you're already using the game via it's standalone ArenaNet launcher, then you'll have to use a new account to use the Steam version, and they cannot be moved across. Conversely Steam version accounts won't be compatible with the standalone launcher.
The game world is shared, however, so players from both versions will be able to interact with each other as normal.
There's also a new expansion on the way, End of Dragons, which will be coming next year. The new expansion sees players head off to the island of Cantha, which has been isolated for over 200 years.