HTML is an evolving language, and each new version is given a number. The first definitive version was HTML 2.0 -- this had most of the elements we know and love, but was missing some of the Netscape/Microsoft extensions, and did not support tables, or ALIGN attributes.
HTML 3 (late 1995) was an ambitious effort on the part of Dave Raggett to upgrade the features and utility of HTML. However, it was never completed or implemented, although many features were integrated in the next "official" version of HTML, known as HTML 3.2.
HTML 3.2 was the next official version, integrating support for TABLES, image, heading and other element ALIGN attributes, and a few other finicky details. HTML 3.2 is the current "universal" dialect -- essentially all browsers understand HTML 3.2. IT was, however, missing some of the Netscape/Microsoft extensions, such as FRAMEs, EMBED and APPLET. Support for these (after a fashion) came in HTML 4.0
However, the evolution of HTML has now ceased -- HTML 4.01 is the last version of HTML. For the future, HTML is being replaced by a new language, called XHTML -- for the eXtensible HyperText Markup Language. The differences are actually small, but important, as described on the next section.