RELATIVE POSITIONS

A line and a circumference can have differents positions:

EXTERIOR	TANGENT	SECANT
It doesn't intersect the circumference at any point because it is outside the circumference. The disrance  between the circumference and the line is bigger than the radius.	They have a point in common. It is the tangency point.  The distance between the line and the circumference is equal to the radius	It intersects the circle at 2 points, so they have two points in common.  The distance between the circumference and the line is smaller than the radius.

Two circumferences can also have different positions:

EXTERIOR They do not overlap and the distance between their centers is greater than the sum of their radius. d= d is the distance between centres r1+r2 > d
TANGENT EXTERIOR They have just one common point, the tangency point.  The distance between the centers is the addition of the two radius r1+r2 = d
SECANT They intersect at two different points and the distance between their centers is less than the sum of their radius. r2+r1< d
TANGENT INTERIOR They have a common point and all other points of one of them are interior to the other, exclusively. r1-r2=d
INTERIOR They have no common points and the distance between their centers is greater than 0 and less than the difference of their radius.
CONCENTRIC  They have the same center and a different radius The distance between the centers is 0 d=0

You can practise and find more information in this link:

http://iessierradesantabarbara.juntaextremadura.net/web/cesar/41_relative_positions.html