Difference between exists() and Region():exists() ?

Hello to all,

could someone tell me the difference between those 2:

if(exists(Pattern("ok.png")

and
ok=Region(1,1,10,10)
if(ok:exists(Pattern("ok.png")

does this mean that it checks the given pattern only at the given region? how big should w and h be? exactly same size as the croped image?
is this faster in searching?

thanks in advance