Marquees are used mostly when making illustrations and selecting type or other regular shapes. The options that are available to you with the marquee include the shape, style, feather radius and whether or not the marquee is antialiased. Option click, double click or press return to get the options floating palette up on the screen. The shape of the marquee can be rectangular, elliptical, single row or single column. Don't forget that you can start a marquee from the middle by holding down the option key.
Rectangular marquees are good for selecting type to kern it.
Elliptical marquees are useful for such objects as plates, donuts, coins and balls. They also make good buttons if filled with color or texture and KPT glass lens filtered or spheroid designer filtered. Single row selections are useful when working with images that originated from video sources and the scan lines don't match up for whatever reason. Single column selections are useful in making grids. Aspect ratios are the width to height values that allow you to constrain to a square or a tv shape or or a picture box shape.