Question about HARD DISK CACHE?

1.well what exactly do the HARD's CACHE memory do??

i mean i do know a little about it but i want some xplantion

2.and if a hard disk has double cache than other one then does it means that the hard with double cache has double speed than other like...

wd 500 gb has 16 mb cache

seagate 500 gb has 32 mb cache

so will seagate perform @ double speed than WD

Most of the benchmarks I have read denote that the performance gains on higher cache is 2-3% on normal desktop usage. They're more useful for servers though.

It's not a linear relation between cache and performance.

The more the better. Its not only cache memory to consider . What about average seek time? and rpm ?