ER Coolet Chucks
They can be used to hold strait-shanked tooling (drills and milling cutters) more securely and with better accuracy (less run-out) than a chuck.
Collets and chucks are two common types of mechanism used to hold spindle tooling or workpieces. Technically speaking, a collet is a type of chuck, but the two terms are usually used exclusively. A 'chuck' usually refers to any type of work holder which is not a collet.