A patch changes something like the string "PAL" to "NTSC". A patch is not necessary to make the game work in its native environment with the modchip in place.

A fix usually deals with anomalies due to variances between external modchips and internal modchips. A fix is a patch that makes the backup play with an external modchip.

A trainer is an "intelligent" patch that offers a menu choice for you to select an option (such as PAL or NTSC.