SDS-PAGE stands for Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis. It is a technique used in biochemistry and molecular biology to separate proteins according to their size/molecular weight.
The proteins are inserted into a (flat) gel and an electrical current is applied. The SDS is added to the gel mixture to denaturate the proteins, to unfold them so they may be separated according to their linear size (i.e., length in amino acids); proteins of similar molecular weights might migrate at different rates due to differences in folding, and folded proteins might be too bulky to move through the gel efficiently. The SDS also provides an even negative charge to the protein (normal proteins may have a wide range of charge distributions throughout their sequence).
Due to their linear form and negative charge, the proteins move through the gel (actually snaking through pores in the gel). Large proteins will move slowly through the gel, constrained by bulk, while small proteins will slip from pore to pore easily and move quickly. Thus proteins may be separated roughly according to size (molecular weight).
Different proteins will form distinct bands within the gel. It is common to run "marker proteins" of known molecular weight in a separate lane in the gel, in order to determine the weight of unknown proteins by comparing the distance traveled relative to the marker.
See also: Western blot