HTML has a number of versions. HTML 4 is the current version. HTML 5 is the new version that is in development but still subject to change as it isn't finished yet.
Most of the sites on the web are written using HTML 3.2 and while the people writing them use a doctype that indicates that the page is being transitioned to HTML 4, very few of the pages have actually been updated to HTML 4 even though HTML 4 has been the current version since 1997.
As HTML 5 is basically HTML 4 with extra tags added (many of which are only there to ensure that browsers continue to support HTML 3.2) the best place to start currently is with HTML 4 (use a strict doctype to indicate that you are not transitioning from HTML 3.2) and then you can add the useful HTML 5 tags when necessary.