Time signatures are expressed as a ratio. The bottom number reflects the value of the note. The top number reflects the amount of notes per measure. For example, 4/4 is four quarter notes per measure, 6/8 is six eighth notes per measure, and so on. Because most music is written in 4/4, all examples are based on that time signature.
The whole note fills an entire measure. In 4/4 time, it represents four beats.
In 4/4 time, the half note represents two beats. It takes two half notes to fill one measure.
In 4/4 time, the quarter note represents one beat. It takes 4 quarter notes to fill one measure.
In 4/4 time, the eighth note represents half of one beat. It takes 8 eighth notes to fill one measure.
In 4/4 time, the sixteenth note represents one-fouth of one beat. It takes 16 sixteenth notes to fill one measure.