Circle
[{$c_x$, $c_y$}, r]($c_x$, $c_y$)
and radius r.Circle
[{$c_x$, $c_y$}, {$r_x$, $r_y$}]Circle
[{$c_x$, $c_y$}]Circle[]
(0, 0)
and radius 1.Graphics[{Red, Circle[{0, 0}, {2, 1}]}]
Graphics[{Circle[], Disk[{0, 0}, {1, 1}, {0, 2.1}]}]
Target practice:
Graphics[Circle[], Axes-> True]