Note value

In music notation, a note value is the amount of time of a note. Note value is relative: it depends on the tempo (speed) of the music. For example, a quarter note (crotchet) at 80 beats per minute (BPM) is the same as a half note (minim) at 160 beats per minute.
Note value is shown by the shape of the note, including the notehead, stem, and flags. Simple note values are fractional powers of two, for example one, one-half, one fourth, etc.
A rest means a silence (no sound) of an equal amount of time.
List.
Shorter notes can be created theoretically "ad infinitum" by adding further flags, but are very rare.
Variations.
Sometimes the longa or breve is used to show a very long note of indefinite amount of time, as at the end of a piece (e.g. at the end of Mozart's Mass KV 192).
A single eighth note, or any faster note, is always stemmed with flags, while two or more are usually beamed in groups. When a stem is present, it can go either up (from the right side of the note head) or down (from the left side), except in the cases of the "longa" or "maxima" which are nearly always written with downward stems. In most cases, the stem goes down if the notehead is on the center line or above, and up otherwise. Any flags always go to the right of the stem.
Modifiers.
A note value may be made longer by adding a dot after it. This dot adds the next briefer note value, making it one and a half times its original length of time. A number of dots ("n") lengthen the note value by its value, so two dots add two lower note values, making a total of one and three quarters times its original length of time. The rare three dots make it one and seven eighths the length of time, and so on.
Two notes of the same pitch can be combined into one note with a tie.
To divide a note value to three equal parts, or some other value than two, tuplets may be used. For example, an eighth note triplet has three notes in the space of two eighth notes, so each note has a length of 1/12. An sixteenth note quintuplet has five notes in the space of four sixteenth notes, so each note has a length of 1/20.