It tells the computer that certain words in the document are bolded, or italicized, or in a column. It can also describe the size of the words and their color. But, unlike procedural computer languages, it doesn't tell the computer to DO anything. I think an example will make this more clear. Let's pretend that sentence is in a web page.
Most tags have a beginning and ending version.
Hundreds of tags exist and new ones are being introduced every day. You'll find a very comprehensive list of tags. Most webmasters create their first web page without any formal training. In act, one of the best ways to learn HTML is to see how others use it. Heck, go to ANY web page.