Moving from Subversion to Mercurial is actually pretty easy. The biggest difference is that Mercurial holds a complete copy the of the repository on your hard drive, while Subversion keeps everything on the remove server.
clone -> "Same thing as SVN checkout, but you do this on your FIRST checkout"
pull -> "Grabs the latest data from the Mercurial Repository, but doesn't apply anything"
update -> "Applies the latest data you've pulled. Kinda like a SVN update"
commit -> "Writes a commit to your version of the Mercurial Repository (on your disk)"
push -> "pushes the latest commits to the central Mercurial Repository"