Different symbols, called notes, are used to tell how long a sound should last. The simplest-looking note, with no stems or flags, is a whole note. All other note lengths are defined by how long they last compared to a whole note. A note that lasts half as long as a whole note is called a half note.
A note that lasts a quarter as long as a whole note is called a quarter note.
In the same way, we can have eighth notes, sixteenth notes, and thirty-second notes.
Here are some common note lengths: