A note is a single sound of music with the parameters pitch and duration.
They are made up of three different components: a notehead, stem, and flag.
In American parlance, a note is called a tone.
Figure 1: Five basic notes.
A notehead is the centre-point of a note which indicates which pitch is played by it's position on the staff.
Noteheads may also indicate a certain technique or instrument to be played.
Figure 2: The notehead outlined in a single note.
A stem is the vertical line of a note which links the notehead and tail together.
Figure 3: The stem outlined in a single note.
Not all notes have a stem.
A tail (or flag) is one or more curled lines at the end of a stem which indicates the time value of a note.
Figure 4: The tail outlined in a single note.
Not all notes have a tail.
Also see Note & Rest Values | Rests | Noteheads (Expression & Technique).