Note duration refers to how long a note should be played. It is represented on sheet music using different types of notes. Some of these are shown here in figure 6.
From left to right on figure 6, they are called the "Whole Note", "Half Note", "Quarter Note", "Eighth Note" and "Sixteenth Note". Each note has half the duration of the note to the left of it. So for example, 2 eighth notes would have the same duration as a single quarter note.
One thing to note about these notes is what is referred to as "Flags", which are shown in red here in figure 7. Each flag signifies that it has half the duration of the previous note. To clarify this, each note has been labeled with its' corresponding duration. There is no real limit to how many flags can be on a note. For example a note with 4 flags is called a "Sixty-Forth Note" and has a duration of 1/64.
Here in figure 8, is a brief overview of notes and their relationship to each other: