Hmm... I think aljoriz means is that the PSP games are more or less the same as the PS version games. The only major difference in interface is that it's portable.
For example - Harvest Moon games. There are HM games for the DS and PSP. the Harvest Moon DS game made use of the DS touchscreen. You can "rub" the animals with the pen (HM is a farming RPG). The mini games also made use of the touch screen. The game had new features because game companies had to alter their game codes to make use of the touchscreen.
The Harvest Moon PS game however is just a remake of one of the Harvest Moon games that game out on the PlayStation. Practically the same game, only that it's modified to play on the PSP.