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 fact, one of the best ways to learn HTML is to see how others use it. Heck, go to ANY web page. Scan through this cryptic mess and you should see many, many tags embedded throughout the text. Believe it or not, most web pages are designed from scratch. Most webmasters simply find an existing page on the Net they like and copy it.