to correct mike j, disc golf discs are rarely heavier than ultimate discs. ultimate discs are typically 175 grams while disc golf discs start at 150 grams, and while they do go up to 180 grams players rarely use anything over 175 and usually a good bit lighter. of all my discs (about 70 of 'em) i only have 2 heavier than 175.
disc golf discs, yes are considerably smaller in diameter than ultimate discs, but their profiles vary depending on their purpose. drivers are flat, with a rather sharp edge, while putters are more domed with a rounded, blunt edge.
also, ultimate discs are 'understable', which means they have a tendency to flip over, or turn (go right for a Right Hand Backhand thrower) while most golf discs tend to be 'overstable' causing them to fade (finish left for a Right Hand Backhand thrower).
lastly, disc golf discs fly considerably farther, but have less of that glide that you're accustomed to with ultimate discs.
hope this helps.
now get out there and throw some plastic!
former ultimate player and disc golfer for 13 years.