A home run is one of the most celebrated and exciting plays in baseball. It is when a batter hits the baseball over the outfield fence, entitling him to run around the bases and score a run. If there are runners already on base, they too get to advance to home plate and score. Home runs (also called homers) are displays of a batter's power and strength. A home run can change the outcome of an entire game with just one swing of the bat.
A grand slam is a special type of home run. They occur when a player hits a home run while the bases are loaded (there is a runner at every base). Grand slams result in the team earning four runs: one for the runner at first base, one for the runner at second base, one for the runner at third base, and one for the batter who hit the home run. In baseball, which is a relatively low-scoring game, earning four runs at once is very exciting and significant. Grand slams are fairly rare, but when they do happen, they are thrilling.
Inside-the-park home runs are another type of home run, even rarer than grand slams. Inside-the-park home runs occur when a player hits a baseball deep into the outfield (but not over the outfield fence) in such a way that he can run all the bases and come back to home plate before the fielders are able to put him out. While typical home runs (that go beyond the outfield fence) are a display of power, inside-the-park home runs show a player's base running speed in addition to their power. Stadium design also contributes to a player making an inside-the-park home run. Parks with deep and irregularly-shaped outfields can allow a batter to hit the baseball into an odd corner of the outfield, making it more difficult and time-consuming for outfielders to collect the baseball.
Back-to-back homers are yet another relatively rare type of home run. They occur when two batters in a row hit a home run. If more than two batters in a row hit a home run, the homers are called back-to-back-to-back... for as many consecutive home runs that have been hit.