What is the difference between alternate and corresponding angles?

See the picture and explanation below.
When two parallel lines are intersected by the third (transversal), they form eight angles: one of the parallel lines forms four angles ##a##, ##b##, ##c## and ##d##, another forms angles ##a’##, ##b’##, ##c’## and ##d’##.
Two acute angles ##a## and ##a’##, formed by different parallel lines when intersected by a transversal, lying on the same side from a transversal, are called corresponding.
So are other pairs (acute and obtuse) similarly positioned: ##b## and ##b’##, ##c## and ##c’##, ##d## and ##d’##.
One of corresponding angles is always interior (in between parallel lines) and another – exterior (outside of the area in between parallel lines).
Two acute angles ##a## and ##c’##, formed by different parallel lines when intersected by a transversal, lying on the opposite sides from a transversal, are called alternate.
So are other pairs (acute and obtuse) similarly positioned: ##b## and ##d’##, ##c## and ##a’##, ##d## and ##b’##.
The alternate angles are either both interior or both exterior.
The classical theorem of geometry states that corresponding angles are congruent. The same for alternate interior and alternate exterior angles.