An error is made when a fielder fails to make a play that he should fairly have been expected to make.
A fly is a hit caught before touching the ground.
A muff is made when a "fly" or thrown ball, striking fairly in the hands of a fielder, is not caught.
A grounder is a hit along the ground.
A steal is made when a base-runner gets from one base to another without the assistance of a base hit or an error.
A wild pitch is a ball thrown by the pitcher out of the fair reach of the catcher, and on which a base-runner gains a base.
A passed ball is a throw by the pitcher which the catcher should stop but fails, and by his failure a base-runner gains a base.
For the purpose of distinction, the nine fielders are subdivided into The Battery, The In-field, and The Out-field. The Battery means the Pitcher and Catcher, the In-field includes the First, Second, and Third Basemen, and the Short-stop; and the Outfield is composed of the Left, Centre, and Right Fielders.
As for the theory of the game, remember that there are opposing sides, each of which has nine turns at the bat, i.e., nine innings, and the object each inning is to score as many runs as possible. A run is scored every time a player gets entirely around the bases, either by his own hit alone or by the help of succeeding batters, or by the errors of the opposing fielders, and the team making the most runs in nine innings is declared the winner. An inning is ended when three of the batting side have been "put out," and a player may be put out in various ways, as before enumerated. The umpire is not trying to be unfair, he is doing the best he can, and instead of abuse he is often deserving of sympathy.