The Default Marketo Fields help article lists all of the fields that Marketo provides out of the box. If you've synched fields from your CRM, you can get a current list of all the fields in Marketo by going to Admin | Field Management | Export Field Names.
Most of the names are self-describing, but a few you'll have to search for in the help articles.
Thank you both for your replies. The list is great but I was more interested in a dictionary if you will for those Marketo fields that aren't self-explanatory. I thought there might be a single source but sounds like i have to keep searching.