The check engine light aka service engine soon light comes on when the on-board computer detects a fault in engine driveability or performance, transmission control system or transmission, or the emissions system, and a fault code is set and stored in the computer. Faults that can set the check engine light can range from a vaccuum leak, engine mis-fire, low engine compression, faulty sensor such as O2 sensor or other engine sensor, hydraulic pressure loss in transmission, transmission shift solenoid, open/short in wiring harness, emission evaporative system leak, or one of over a hundred different reasons.
The only way to know what set a fault code and turned the check engine light on is to have a diagnostic scanner hooked up to the on-board computer and read it for stored diagnostic fault codes. Many auto parts stores(like auto zone) will do this service for free- check with you local parts stores. If none in your area do, then you will have to take it to a shop and pay to have the computer read for stored fault codes.