(Note, this isn't official policy, it's just an observation of how things have been done the past few revs.)
Depending on the server type, admins will periodically post cartographs of the current revision to the subreddit:
- Creative - cartographs can be created whenever.
- PvE - admins generally wait a couple weeks before releasing any cartos, in order to let everyone explore the terrain.
- Survival - no cartographs for the current revision.
As of June 2012, Creative/PvE cartos for the current revision are automatically generated every couple of days.
The save file for current revisions is not available to players, so you will not be able make your own cartograph. Once the rev is complete, however, you can download the save from nerd.nu/backups. The wiki server hosts cartos of past revs as well, see Minecraft-overviewer for a list.
Cartograph is one of the original tools created used to map Minecraft worlds. The popularity of the program has meant that 'cartograph' is now commonly used as a general term for a map generated from a minecraft savefile, even if created with a different mapping tool. Since mid 2012, nerd.nu has been using Minecraft-overviewer to generate cartographs.