cache is a fast accesing memory located in the HD that could help you get things faster. once you pick out a thing in the harddrive it'll be stored in the cache. when you search the next time the HD would search the cache first then if it's not located there then i'll actually search the HD. of course bigger cache size is better but only for big HDs, small HD might not find it very useful. you might want to get a bigger one for the task you're doing. but they come in price too.
I am the yellow one.