Batters combined only get to see 120 "pitches" and have to score as many runs as possible in that time.

If your batsmen are all out (10 of the 11 players get out), that could end it early, but it's pretty unusual for that to happen before 20 overs. As a result you'll see batters be more aggressive than they would in "test" cricket which has no limit on the number of overs.

Technically they could also bowl "wide" or other types of "no ball", which is like an extra free ball that doesn't count toward the 6 in an over. After each over, they switch bowlers and change ends on which direction the bowler is delivering from. There are also rules on the number of overs a particular player can bowl, etc.