All characters connect to the Trieste server when entering the world of Dekaron, but you can switch between three separate instances of the world, known as channels. Channels are useful if you are looking for a less crowded area to quest and farm, want to avoid PvP combat, or if you are just tired of seeing the same faces.
To switch channels, open the Options menu (ESC) and select Channel. You will be shown the three available channels, with your current channel grayed out. Be advised that switching channels will remove all your temporary buffs, and your pets will need to be re-summoned.
The Divine Valley and Kalibus channels are functionally identical, but the nonPK channel has a restriction: you cannot engage in PvP combat anywhere out in the world, except for agreed-upon duels. You cannot enter DK areas while you are in the nonPK channel. Another channel, the PK channel, differs in that you can engage in PvP combat without penalty - that is, attacking and killing other players does not confer IP or Niete status. The PK channel also features bonus experience gain and item drop rates.
You cannot switch channels while in DK areas - all players on a server share the same channel for these areas.
If you are trying to meet up with someone at a particular location and cannot see them, they may be on a different channel, in which case they will not be visible to you. Try switching channels and see if they appear to you.
You can always enter the world in a particular channel, if you prefer, by selecting it on the Character Select screen.