Namur: The main difference bewteen URU:CC and MOULA - which is the free version you mentioned - is that URU:CC is a single player game while MOULA is online mutliplayer - you will need to cooperate with other players to finish some of the puzzles and stuff. There are other differences but most of them are not really that important.
Trying to keep it spoilers free,
If you read the books then you know that the D'ni civilization eventually crumbled. URU takes place after a group of explorers found the remnants of URU - the ancient city. Eventually those explorers also left, and now it's you, the player, who is about to explore the Deep City. You'll come across lots of interesting things not only concenrning the D'ni, their history and their Ages but also about the group of explorers that came before you.
thanks for the quick reply but is it the same engine both games play. Because the online version was a bit on the dark side some things i couldnt even see. And i think i want to try real meast since its been a while i read the books does anyone know if myst 1 is on the same time as the books? I remember that is about 2 sons who created there own worlds red and blue one wich also was the books i red.