Swap is, in short, virtual RAM. With swap, a small portion of the hard drive is set aside and used like RAM. The computer will attempt to keep as much information as possible in RAM until the RAM is full. At that point, the computer will begin moving inactive blocks of memory (called pages) to the hard disk, freeing up RAM for active processes. If one of the pages on the hard disk needs to be accessed again, it will be moved back into RAM, and a different inactive page in RAM will be moved onto the hard disk ('swapped'). The trade off is disks and SD cards are considerably slower than physical RAM, so when something needs to be swapped, there is a noticeable performance hit.
Unlike traditional swap, Android's Memory Manager kills inactive processes to free up memory. Android signals to the process, then the process will usually write out a small bit of specific information about its state (for example, Google Maps may write out the map view coordinates; Browser might write the URL of the page being viewed) and then the process exits. When you next access that application, it is restarted: the application is loaded from storage, and retrieves the state information that it saved when it last closed. In some applications, this makes it seem as if the application never closed at all. This is not much different from traditional swap, except that Android apps are specially programed to write out very specific information, making Android's Memory Manager more efficient that swap.
This question is hotly debated, but you almost definitely do not need swap. The only exception to this may be if the device is a first generation device (i.e. HTC Dream or HTC Magic).
Swap can give more available memory, however, class 6 SD cards are recommended and SD write wear is increased.
Actual performance depends on user memory use; you'll only see a benefit if you're consistently using up all available memory, due to any combination of inherently low device RAM, using multiple apps simultaneously, or a singularly memory-intensive app. Otherwise, the performance hit will exceed any performance gain.
How can I tell if swap/compcache is running?Go to the terminal emulator - or open adb shell - and run 'free'.
If it looks like this (with zeros in the swap line), you do not have swap
total used free shared buffers
Mem: 97932 96640 1292 0 272
Swap: 0 0 0
Total: 97932 96640 1292
Alot of the present roms floating around right now,don't use the swap partition,but its a good idea to leave something like 1GB or less for future swap initiation.