1) You would need to play Myst in order to understand what happens in Riven.
2) Myst - Pros - This was a great game and ahead of its time period. Wonderful story line. Acting is done reallly well
Cons - Puzzles don't really have anything to do with, well, anything, but they are still fun to solve.
Riven - Better graphics, good story line, long hours of gameplay to complete without guide. Plenty of different places to explore. Once again, acting is done even better.
Cons - Puzzles can get frustrating, but what's a good game without a little challenge.
3) I like Riven better, but I highly suggest you get Myst: Masterpiece Edition. It is the same game as Myst, but it has a built in guide and these games are really cheap since they're older games.
I know I contradict myself above, but hopefully this helps you.