Almost everyone knows what an eraser does. It removes a section of a picture. It can be very easy to use if used basically, or very complicated if used to its full potential.
The Tool
Start by clicking the eraser tool 
. Then scroll over to the Image and click + drag to remove portions of the image. It can be that simple.
Complicated Options
You already have seen how to use the basic eraser. But this tool can do much more then just remove squares.
Mode
The mode option sets the eraser to act as a block, a brush, or a pencil. A block acts as a simple square eraser, a brush acts a brush with varying opacity, and a pencil acts as a 1px*1px eraser.
Brush
The brush, unlike the other options, requires a little more explanation. Next to "Mode" you will see the "Brush" option. This allows you to set the shape. This is explained further in the Brush lesson.
Opacity
This sets how much it erases off the picture. at 100%, it removes everything it touches, and at 0% it removes nothing.