Pretty much a runescape private server is a rip off of different time periods of Runescape. They are generally more buggy and have very little content. Here is the hub of private servers were you can find sources and clients:
http://www.moparscape.org/smf/index.phpYou can pretty much host your own version of Runescape
Pros -
-Fun to play around with (hosting your own, making your own content, custom NPC's and objects learning how the game works) For example I created a bootleg Zaros GWD in like 2011. I took a black unicorn, made it hit harder, changed it's name to "saturn", changed its level to "139", then took the chaos elemental, renamed it Zaros, ajusted hits, used a couple other NPC's (forgot which ones) and then put them all inside a dungeon. "Zaros" dropped a zaros hilt which was a rotated ancient staff sprite cut in half, which could be combined to make a "zaros" godsword which was a Zamorak godsword with the name changed.
-Fun to play(if your poor on RS)
-Can play previous versions of Runescape
-Different custom content then RS
-No bot detection
-Generally more responsive staff
-Good place to start out making simba scripts (I did this for a while)
Cons:
-Corrupt staff
-Duping/major in-game glitches
-Can be easily hacked if it does not contain the proper failsafes (abunch of packet stuff)
http://www.moparscape.org/smf/index.php?topic=551310.0 (I have full on spawned items on a VERY bad server I have a vid of it I can upload if you want)
-Unpopular ones may contain virus's (this is actually rare..)
-some are Laggy
-Generally bad in-game economy - most servers run off "donations" when someone donates they get in-game items causing a inflated economy
-You can get your computer memory raped if you don't use a server with anti-syi (or else someone will use allgofree bot makes like 1000 player accounts in like .5 seconds each player account is like 23 kb)
-You can get your computer infected hosting a server from your home computer also.