Two objects are congruent if they have the same shape, dimensions and orientation. Congruent circles are circles that are equal in terms of radius, diameter, circumference and surface area. Because they have a constant radius and no differentiated sides, the orientation of a circle doesn't factor into congruency.
Any circles that have the same radii can be overlaid perfectly on one another.
More complex objects have to meet more parameters to be congruent. Polygons must have the same number of sides arranged in the same shape with the same dimensions and must be oriented the same way. Even if two rectangle have the same dimensions and shape, if one is oriented horizontally and the other vertically, they are not congruent.