Paragraphs are bounded by <P> at the start and (optionally) </P> at the end.
Of course, lots of folks just begin paragraphs and don't worry about ending the previous one. That's fine. It works. Part of why/how it works is that lots of things in effect end paragraphs. Things like lists and ``Horizontal Rules.'' Here ``Rule'' just means a straight line as one sees below.
The general format of bounding something by <XXX> at the start and </XXX> at the end works for setting physical styles of print as well: