Look at the file type in your iTunes listing.
Can't remember exactly when but in either 2008 or 2009 Apple stopped adding DRM protection to music files sold via iTunes. Files from prior to this will be marked as "protected AAC" and those post "purchased AAC". The former are linked to authorised devices the latter can be freely shared irrespective of account. Any protected files can also be upgraded to the non-protected version (called iTunes plus) via iTunes so you can have all your purchased music free of DRM.
All video purchased via iTunes is protected and has to be linked to an account on the five machine rule per Ghosty's post.