What happens when I unsubscribe from iTunes Match? I know that for every song I have purchased from the iTunes store I can download it regardless and at any time. I was thinking of subscribing to iTunes Match to get a similar service for all the songs I have bought from other sources. I was thinking though, if I unsubscribe at a later date will I no longer be able to download the songs that I didn't purchase from apple through iTunes?

Or will all my songs that are recognised by "Match" be classed as purchased through iTunes and available to download indefinitely?
No, iTunes distinguishes your content gathered from outside the iTunes Store as either Matched up Uploaded. Matches means if found the song on iTunes while Uploaded means it could find the selected song but instead took your copy and placed it in the cloud. The benefit of the former is that Apple's copies are high quality AAC files and will replace your sometimes inferior lower bitrate MP3 in the cloud. At no time does the service concert any item to the "purchased" status however.

Once the service expires, Matched and Uploaded songs no longer can be downloaded indefinitely. The iTunes Match service is essentially the download portion of the service. Once you unsubscribe, you can no longer download any more songs from the service that have been marked as either Matched or Uploaded. Any song you downloaded while iTunes Match was active, you get to keep, though.

More information on iTunes Match can be found here: http://support.apple.com/kb/ht4914