Well, the answer to this question should be to read the crickets rule book! However, let me elaborate few points which helps someone understand the game of cricket. (the shortened version, TwentyTwenty)
The usual cricket game existed in two different forms, the one day game and the five day test cricket. In recent times, there had been a drop in spectator turn-out and the sponsorship revenue gained from these forms of cricket. Thus, a new version of cricket was introduced which is the twenty twenty cricket lasting only 40 overs, 20 each for each side.
In T20, each side will get 20 overs with each over having 6 balls each (as in any other form of cricket). Although in one-day cricket, each bowler can ball 10 overs, in T20, each can only ball maximum of 4 overs. There limited number of overs where field restrictions are in place.
The umpires job is actually to signal the overs, decide if a player is out when the fielding side appeal, detect no-balls or instances where the bowler overstep the line or intervene in instances where the rule book has been violated.
When looking at players, I would think Lasith Malinga is the best fast bowler in T20 while Peterson from Englans, Jayawardena and Dilshan from Sri Lanka, Guptil from New Zealand, Dumini from South Africa and Raina from India are the best batsman.
Hope this answers your question to an extent.