Yup, the Caves from Nexus in the main hall (with monster names) are server instances. There is a max concurrent population of 85-players on any given server instance at any given time. Join a full one if you want to find a nice group to grind with, join a light one if you are having lag problems.
PRO TIP: F5 is the default hot-key to return instantly to the Nexus at any time. This will save your ass in a bad fight that is going south. Most players recommend you bind the return to Nexus to a different key. Personally I use left-CTRL, it is a lot faster to hit with wasd-fingers than to reach for F5. Map your hotkey and practice it so you can mash it in a flash without having to think about it, and you will keep your characters alive a lot longer.