A private server is an unauthorised version of RuneScape Classic that is not run by Jagex. Such servers are illegal as they violate copyright. They are generally looked down upon by members of community due to the risk of malware, cheating and hacking that is associated with private servers. Thus, Jagex strongly recommends against using such servers.
Private servers also often suffer from bugs that were not present in the original RuneScape Classic, such as odd non-player character movements or monsters dropping incorrect items or nothing at all. They can also be modified heavily and differ from the official game.