If you work with a computer, chances are you have a screenshot grabber that you use to prove a point. You can attach it to email and such. As for the tablet, there is also such a thing.
The interface is user friendly, you can get by without any help. I usually just do a screenshot and edit it with it. One of my recent blogs utilized this fully.
It basically has the basics covered, an arrow, changeable colors, free hand drawing and shapes.
Verdict: For a free app, this tool is really handy when you are on a tablet more often than a pc. It's not like as if instagram can help you point out certain areas in your photo right? 4 out of 5 stars, 5 if they update the crop option. I hope I am able to control what to crop.