Quarter note
A quarter note (in British terms a crotchet) has a filled-in head and a stem and lasts one beat. Four of them fill a measure of 4/4.
The quarter note is the basic counting note: one quarter note, one beat. The bottom number 4 in a time signature such as 4/4, 3/4 or 2/4 means exactly this — the quarter note gets one beat. Four quarter notes fill a measure of 4/4, three fill a measure of 3/4.
Compare it with the other values: a half note lasts two beats, a whole note four, and an eighth note only half a beat, so two eighth notes fit into one quarter note. The silence of the same length is the quarter rest: one beat without sound.
The name comes from division: a quarter note is a quarter of a whole note. In rhythm syllables a quarter note is read ta.
What it sounds like
Quarter notes sound like an even walk: one sound on every beat. That is “ta, ta, ta, ta”.
Where it is taught: the studio’s interactive textbook, chapter 2, “Counting: the beat, quarter and half notes”.