If you meant the ID 15 connection error, click here.
User ID[]
A user's ID represents the if they're the 'XXth' user. For example, a user with the ID 4000 would have been the 4000th person to join the site. These were once highly competitive, with ID sniping becoming a lot more common. They essentially were a tag of when you joined, and existed as a reminder that you were one of the first people around or at least got lucky and had a cool number.
As of an unknown date, ID's are no longer displayed in profile URLs with the release of the new site layout, but can still be found by viewing the ID in the image URL of a user's avatar. This update does mean, however, that ID surfing is pointless unless you, for some reason, want to surf avatar ID's, and you also can no longer figure out what the username of an account with a certain ID is, but it might be possible to search the avatar image on google and see if the GT profile of the user comes up.
Media[]
The user's ID in the search bar, before it was removed.
The replaced user ID, now with the user's name instead.