Adding the mktNoTrack class to an email link tells Marketo not to add a tracking link to the URL. Without the tracking link, the recipient is not redirected through the tracking server before going to the target URL. Use this when you specifically don't want to track Clicked Link in Email events in the Marketo database.
Adding the mktNoTok class to an email link tells Marketo not to add the _mkt_tok parameter to the target link. Used when the target link does not behave properly, for example, a mailto: link that should not have extraneous URL info or a static file that won't download when there are query parameters. The click activity will still be tracked, but the associated lead info will not be carried onto the page for use in functions such as form prefill.