In Neverwinter, there is currently one live Shard called Dragon, representing a single game world with specific guilds/auction/etc, as well as a preview Shard called Mimic. Within each Shard however, there can be many temporary instances of each map. These are numbered in the following format: Black Water District #18. This does little to affect game play, unless you play regularly with a friend or group of friends. When you enter the game, you are automatically put in the "best" instance, presumably the one with the least players or highest number of guildmates.
You can change to another instance, but there will usually be a delay of up to two minutes after login or changing zones.
If you wish to play with friends that aren't in your instance, you may "Transfer" to their instance in the following manner.
- Go to the Social Window, either by pressing the hotkey (default "O") or clicking it up at the top of your screen.
- Find one of your friends on the Friends Tab.
- Right Click on the friend's name, which should show the name and number of the appropriate instance (example: Black Water District, #18)
- Select "Transfer to map instance."
- Assuming you are not required to wait, you will be prompted to confirm your Transfer, and then you will load in.
Alternately you can switch to any existing instance of the current map my using the Change Instance button in the bottom right corner of the large map dialogue window.