The classes simply specify what set of hardware is used (the server that hosts the sim) - the higher the class, the better sim performance. Here's what Ian Linden wrote about the differences, when Class 5 was introduced:
From a resident perspective, the key difference is that the newmachines should offer higher, more consistent sim-side performance forheavily-loaded regions, especially when there are a large number ofscripts running. LSL performance should be quite a bit higher, althoughwe won’t know by exactly how much until Class 5 has been running inproduction for a while. More generally, the new servers have somefuture-proofing features for us, and use less electricity than previousgenerations.
So, here’s what’s different under the hood: we’vebeen all-AMD for years, but are moving from the Opteron 270 to theIntel Xeon 5148 - a low-power version of Intel’s new Core 2 Duo basedserver CPUs. This gives us better performance for fewer watts, whilesupporting our standard 64-bit OS image. We’ve also doubled the RAM permachine from 2GB to 4GB and moved to a faster SATA disk, which usuallywon’t make much of a difference, but should reduce the stalls sometimesseen by heavy regions during autosaves. Finally, there are fewer,bigger system fans, and power supply efficiency goes from 67% to 84%;power usage while running the sim process is about 175 watts, vs. 230for a Class 4.
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