lasso tool

lasso:

a lasso can be used to do freehand drawings with your mouse.

it also can be changed into different formats e.g. you can cut out one main object in a picture and change on not have a background.
the other format magnetic lasso tool can be used to curve corners.it's useful for copy and pasting. 

here is a link to a tutorial on how to use the lasso tool on adobe photoshop.

http://www.youtube.com/watch?v=5AqwrAJYf5Q