Marquee is used when you want to make a square/rectangular shape on your image. That’s the definition I always give when talking about the marquee tool. But marquee is not only use to make a selection for square or rectangular shape. Marquee comes in different shapes:
This is the one I always use. You will use it if you want to make a rectangular shape selection into your image.
This is a marquee tool that is use when you want to make an elliptical shape of selections.
Single Row marquee
It will make selection to one row of your image. This marquee will create 1 pixel high – horizontal selection on your image.
Single Column Marquee
It will make selection to one column of your image. This marquee will create 1 pixel wide – vertical selection on your image.
Note: You can try them out by clicking a part of your image >> holding your mouse >> move
You can see a formation of marquee tool depending of what marquee you are using.