There are a lot of huge differences between live and online, both in cash games and tournaments. The first obvious difference is that you have to physically go to a card room or casino to play live. It may sound a little ridiculous to point that out, but, if you think about it, you are taking time to choose an outfit, drive to a destination (at 5 bucks per gallon...just saying) and sit in a different environment where you don't have to luxury of toting around a laptop if you need to go to the bathroom or want a snack. There are also limited options on what you can play in a card room or casino, whereas, online, you have pretty much a whole world of options to choose from and you can have multiple games running at once. Live, you are at one table.
Next, you have the opponents. They are also people who made the above decisions, the main difference is, they have come to play and not fold. So, you will run into much looser opponents (I know this sounds crazy, but it is true) in live games than online. This is for a few reasons: they have come to play, there are fewer hands per hour so they are forced to play a wider range to "get in the action" and most of them do not play a lot of online poker so they are not as strong. That's why a .10/.20 game online is actually pretty comparable to a real world $1/2 game.
Further, in live games, you and your opponents are physically present. This leads to having live reads and picking up tells. This is HUGE especially if you pay attention and pick up on these tells while trying to minimize your own.
Finally, live poker is pure. There are no RNGs to determine outcomes. Sure, there are still bad beats, but they adhere to the mathematical probabilities that you would expect. Online poker does not (even factoring in the extra hands per hour and all the other excuses online poker uses), the fact is that there is no present way to simulate the actual shuffling and dealing of the cards. But, at least in online, you don't have to tip the dealer and the cage. LOL.
This is just a short list. If I had time to really examine it, there would be a much more detailed explanation. But, I am sure you get the point that online and live are really two different games and should be treated as such.