At the beginning of each piece of music, after the treble clef, is the time signature.
The top number indicates the number of beats per bar (in this case four) and the bottom number indicates the type of note receiving each beat (in this case a quarter note).
Thus in * time there must be the equivalent of four quarter note beats per bar.
* is the most common time signature and is sometimes represented by the common time symbol.
^ (pronounced Three Four time) indicates three quarter note beats per bar.