See
http://www.bandcexpress.co.uk/ Chain links come in different sizes, so you need to match your chain to your sprockets (or vice versa) to ensure they all work together correctly.
There are various different dimensions to consider - chain pitch (the distance between the rollers), chain width (distance between side plates) and the roller size being but three of them.
For a 520 chain, the pitch (p) is 15.875mm, width (w) is 6.25mm and the roller distance (Dr) is 10.16mm
For a 525 chain, the pitch (p) is 15.875mm, width (w) is 7.75mm and the roller distance (Dr) is 10.16mm
So while 520 and 525 chains are very similar, it's the distance between the sideplates which differs, so you need thicker sprockets - a 525 chain usually goes on more powerful machines.
Ducati use 525 chains on all their current models for economy and standardisation. The 748 (except the R) has a 520 chain and the 916+ have a 525 chain. I believe the 600 line also had a 520 chain as standard, but it should be stamped on the side plates to tell you what it is.
The reason for using a 520 chain over a 525 is that a 520 chain will be lighter than the same size 525 chain, and with therefore reduce the amount of power taken by the drivechain (as a heavier chain will take more effort to move by the engine) giving you more power at the back wheel. Ok, so we're only talking hundredths of bhp here, but every little helps
