Thanks hectictar, i'd just like to add a little more.
\((-4)^2=-4*-4 =+16\)
Maybe that one is fairly obvvious to you..
If not then look better at Hectictar explanation.
\(\text{So why is }-4^2\;\;\text{different ?}\\\)
Welll... Lets say that -42 did equal +16 (which is doesn't) and see what happens
\(30-4^2=30-16=14 \)
But
\(30-4^2\;\;also\;\;=-4^2+30\)
If -42 = +16 then this answer becomes 16+30 = 46 which is obviously wrong because I know the answer is 14 !!
so the best way to think of it is this
\(-4^2=-1*4^2=-1*16=-16\)
I hope that, together with Hectictar's answer, helps :)