The duration of a note is determined by three things: the note head, stem, and flag. A whole note equals four beats. A half note is half of that: two beats. A quarter note equals one beat, an eighth note equals half of one beat, and a 16th note is a quarter beat (there are four 16th notes per beat).
The fraction (4/4, 3/4, 6/8, etc.) or 
character shown at the beginning of a piece of music denotes the time signature. The top number tells you how many beats are in each measure, and the bottom number indicates the rhythmic value of each beat (4 equals a quarter note, 8 equals an eighth note, 16 equals a 16th note, and 2 equals a half note). The most common time signature is 4/4, which signifies four quarter notes per measure and is sometimes designated with the symbol (for common time). The symbol 
stands for cut time (2/2). Most songs are either in 4/4 or 3/4.
