The new server will be used to run a 2nd copy of the runescape world to reduce the overcrowding. The 2nd world will be exactly the same as the current one, and so will effectively allow twice as many people to play Runescape at once.

When logging in you will be able to choose which server to play on, and will still be able to private message your friends even if they are playing on the other server. The existing saved games will work on either world.