They are completely different. Disruption means you have already enlisted, and are putting your NS on hold till you complete your studies. Most of the time this is for medical students, those that applied for medicine at either NUS or NTU, and are waring for acceptance. But in the mean time, if they receive their enlistment letter, they have to enlist first.
Another time you can disrupt is for Uni Studies towards the end of your service. Say you are starting Uni in August, but you ORD in Oct. If you have served anytime between 20-24 months, you may disrupt for Uni studies. (Just that you will have to serve back the remainder during your holiday break from Uni, some people do this as it is better than waiting another year for the next Uni batch)
Deferment on the other hand, means you have yet to enlist, received your enlistment letter or not. You may be deferring you complete further studies. (up to tertiary education) This is mostly for people who are coming out from 'O' levels, and are asked to defer their NS till they complete their JC/Poly course; or for Nitec students, who wish to progress on to higher Nitec, then to Poly.