A crotchet (also known as a quarter note) is a note that lasts for one beat. A quaver (also known as an eighth note) is a note that lasts for half a beat.
Crotchets and quavers are often used together to create rhythms. For example, a common rhythm is a crotchet followed by two quavers. This rhythm is often used in marches and other upbeat music.
Crotchets and quavers can also be used to create melodies. For example, a melody might consist of a series of crotchets and quavers played in different pitches.
The length of a crotchet or quaver can vary depending on the tempo of the music. In a slow tempo, a crotchet might last for several seconds. In a fast tempo, a crotchet might only last for a fraction of a second.
Crotchets and quavers are just two of the many note lengths used in music. Other note lengths include whole notes, half notes, and sixteenth notes.