A slanted line (a transversal) crosses two parallel lines. Change the angle and pick vertical angles, corresponding angles, and alternate angles one at a time to check them. As long as the lines stay parallel, these relationships always hold.
Vertical angles are the opposite angles formed where two lines cross at a point; corresponding angles are angles in the same position at two different intersection points; alternate angles are angles on opposite sides of the transversal, between the two parallel lines. When two lines are parallel, angles in each of these three relationships are always equal in size.
These angle relationships are hiding all around us. A railroad's two rails are always parallel, and the angles the railroad ties make crossing between them are exactly corresponding and alternate angles. You can find the same principle in a building's window frames or the parallel shelves of a bookcase.
Use the angle slider to change the transversal's slope, and press through the three angle relationships one at a time to see which angles pair up.