The
selection tools allow you to select a portion or a subset of an image to
work on. Most of Photoshop’s other tools and filters can then be applied
to this selected area, altering its color, shape, texture, position
and/or other attributes, while leaving the rest of the image untouched.
Photoshop contains three types of selection tools: the
Marquee Tools, the Lasso Tools, and the Magic Wand.
The marquee tools are used to select a specific
regularly shaped area. The marquee tools include the Rectangular,
Elliptical, Single Row and Single Column Marquees:

The lasso tools are used to select an irregular area.
These tools include the Lasso Tool, the Polygonal Lasso Tool and the
Magnetic Lasso Tool.

The Magic Wand
is used to select
areas of an image based upon color; the Magic Wand does not have
additional tools.
All of the selection tools may be used individually or
in conjunction with each other to select exact areas of images.