Servers make up Second Life's grid in which the client connects to. "Server" can sometimes refer to the software that runs a sim or more typically the actual hardware that the sim runs on. Each server (hardware) may host anywhere from two to four different sims (software). The hardware most used are "Class 4" servers, running Debian Linux, and containing 2 dual-core CPUs with each sim getting the equivalent of one CPU (4 sims per class 4 box).
Previously Linden Lab used "Class 2" (1 CPU, 1 sim) and "Class 3" (2 CPUs, 2 sims) hardware types, but these were phased out and should no longer be in use for most sims (exception being possibly test sims on preview grids).
- LSL Wiki: server
- LSL Wiki: simulator - Details on server hardware are at the bottom under "Technical Specs".
- SL Forums: Lee Linden gives some details on hardware while discussing per-sim usage
- SL Forums: beez Linden posts information on the progression of converting all hardware to class 4 types