it's an instant messenger much like gaim, meaning it supports the most often used protocols in one client. it's advantages to gaim are that:
1) you can see your own status, and that for every protocol seperately
2) it blends nicely with kde
3) you can show osd message alerts, great for machines without sound
4) it has a lot more features: http://sim-icq.sourceforge.net/features.shtml
duh, i could sewar i searched the whole bloody forum...