Basic Commands
hg branch - display the branch you're currently on.
hg branches - display all branches for the current repository.
hg checkout <branchname> - checkout the specified branch. Also if you want to disregard any local changes before switching use the --clean option.
hg status - show all files that have changes pending.
hg diff - output the pending changes of all files.
hg commit -m <commit message> - commit pending changes to your current branch.
hg pull - get the most recent changes done to the repo, from the remote server.
hg update - this will attempt to merge all the changes from the remote pull, to your local file system.
hg push - push all your changes up to the remote server.
Extra Commands
hg serve - by default, will start a server on localhost:8000, which will give you a neat little UI in your browser to view your mercurial logs, branch graphs, tags, bookmarks, and branches. It also has a help section to get more detailed info on hg commands, and whatnot.
hg graph -r <commit revision> - when you want to pull in a specific change revision to your branch without pulling anything else (similar to git cherry-pick). Also depending on your Mercurial version, you can also use hg transplant more info
hg shelve and hg unshelve - set your pending changes aside temporarily and restore them at a later time. more info