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Gel Electrophoresis
All blotting techniques start with gel electrophoresis. The gel is more or less like a thin rectangle of gelatin. A tiny amount of a specially prepared biological molecules is put on the gel at one end, then an electric field is applied from one end to the other. The prepared molecules are electrically-charged, so the electric field pulls them through the gel. They move slowly through the thick gel until the field is turned off. The lighter molecules get pulled faster and move further, the heavier molecules take longer to move so they don't make it as far.
Using Gels
In practice, several different samples are put down spread out along one edge of the gel. After the electric field is applied, the different samples spread out heading for the opposite edge of the gel. The spread of each sample is called a "lane." The lane will usually have a few different bands corresponding to molecules of different weights within each sample. There are several different ways of making the bands visible. Analyzing the gel gives biologists an idea of the size of the different molecules, but to get more information, a different method is needed. That's where blotting comes in.
Blotting
Blotting is a way of taking the molecules that have been spread apart in the gel and making them available for more analysis. In the same way blotting paper soaks up loose ink --- or did in the days when folks used fountain pens --- biological blotting "soaks up" the molecules from the gel into a different material. The molecules end up in the same pattern, but now in a special paper or plastic sheet. The molecules get bound to the sheet so they don't move any more, and then they can be soaked in a solution of other probe molecules that provide more information about the molecules. There are several different ways of doing those steps, but the principle is the same for all of them.
Southern, Northern and Western Blotting
Biologist Edwin Southern was the first to develop a blotting technique. He developed all the details necessary to be confident that DNA gel electrophoresis could be accurately transferred and measured in blotting material. That method was named after him: Southern Blotting. Other scientists figured out how to make the same general technique work for RNA, and --- in typical fun-loving scientist style --- called their detailed method Northern Blotting. Later, the procedure was modified for measuring proteins, and was called Western Blotting. All three techniques use the same principles: Southern measures DNA, Northern measures RNA, Western measures proteins.