Circle[Point M, Number r]: Yields a circle with midpoint M and radius r.
Circle[Point M, Segment]: Yields a circle with midpoint M whose radius is equal to the length of the given segment.
Circle[Point M, Point A]: Yields a circle with midpoint M through point A.
Circle[Point A, Point B, Point C]: Yields a circle through the given points A, B and C.
Use circles to fix the distance between two objects
Circles are a great way to make the distance between two objects constant: If there are two points A and B on two lines g (point A) and h (point B) where A can be moved and B should have the constant distance r to A you can define B as the intersection between the line h and the circle around A with the radius r. As a circle intersects a line at two points (in case it's not tangetial or passing by) you have to hide & ignore the second intersection.