Sodium DodecylSulfate- PolyAcrylamide Gel Electrophoresis is a technique used to separate samples of proteins by size by unfolding the proteins & coating them with a uniform negative charge and then using electricity to send them them traveling through a thin vertical slab of gel mesh.
Each gel is basically a tiny molecular race in which proteins, coated with a negatively-charged detergent called SDS, push past one another (within their lane), slither through the gel’s pores and near the positively charged “finish line” at the bottom of the gel. Bigger (in this case, longer) proteins get tangled up more and thus travel more slowly. The smaller the protein, the easier the journey, so when you stop the race on demand by turning off the electricity that was creating the charge gradient, the smaller proteins are lower down (nearer the finish line). You can see this when you stain the gel (such as with coomassie brilliant blue) – proteins will show as bands. By comparing to a ladder of proteins of known size (molecular weight markers) you can tell how big the various proteins are. Additionally, by looking at the number of other bands (representing other proteins in the race) you can also get an idea of how pure the sample is.
SDS-PAGE does not tell you about the identity of the proteins, but you can transfer the length-separated proteins horizontally out of the gel and onto a membrane and use antibodies to probe for specific proteins. That’s called a western blot.
If you leave out the SDS, the proteins remain folded and you can separate them by “shape” and not just size. This (with some other modifications) is called native PAGE.