Swap vs. memory

Not sure if this is the right forum but here it goes. When I type top in my dedicated server it shows me CPU usage, mem usage and swap usage. I realize that swap must be a type of memory but what exactly is it? And how is it different from regular memory. Here are my current stats

CPU states: 23.4% user, 2.2% system, 0.0% nice, 74.4% idle
Mem: 498144K av, 487676K used, 10468K free, 0K shrd, 21568K buff
Swap: 1012052K av, 0K used, 1012052K free 273856K cached

 

 

 

 

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