Selection Tools
When we need to edit a portion or complete object we must select that portion to point out which portion is being modified because PHOTOSHOP doesn't know which portion of image you want to change.
This is a set of selection styles:                         
When you click and hold your mouse on the rectangular selection following options will appear :
ELLIPTICAL MARQUEE TOOL : To select oval or perfect circular portion of any size.
RECTANGULAR MARQUEE TOOL : To select rectangular or perfect square portion of any size.
SINGLE ROW MARQUEE TOOL : To select a horizontal linear area of one pixel thick.
SINGLE COLUMN MARQUEE TOOL : To select a vertical linear area of one pixel thick.
CROP TOOL : To change the size of image by cutting method.