In short
A circular junction at which traffic flows anticlockwise (UK) or clockwise (US); give way to traffic already on it.
A roundabout is a circular junction at which traffic flows anticlockwise in the UK (clockwise in the US and other right-hand-drive-traffic countries). The basic rule: give way to traffic already on the roundabout, choose your lane on approach, and signal your exit one before the one you want.
Roundabouts are a high-fault area for learners. Hesitation at a clear roundabout is one of the top three marked driving faults — examiners read it as a planning failure, not a safety win.
See also
Related terms