Yeah its the "prologue" version of GT5. Which means its giving everyone of a taste of whats to come, GT5 will be released later this year and that is the "full game".

The fact is though, GT5 Prologue is well worth the cheap RRP you will pay for it. It has online multiplayer and will recieve DLC (Downloaded content) in the future. So its not really a "prologue" to GT5 in a sense.