Best Snippet Tool for MacBook Air

I need a tool to take a quick rectangular photo to help illustrate what I am referring to when working.

I don't need an entire screenshot, just something quick and easy that I can grab exactly what I need and nothing more.

Microsoft has the "Snipping Tool" and it's so simple and easy and I am having the hardest time finding something like that for Mac. If anyone uses something similar to that affect or better please let me know, thanks!