ASA means two angles and an included side are equal. If you have triangles ABC and DEF congruent, then ASA would apply if angles A and B are equal to angles D and E, respectively, and the included sides, AB and DE are equal.
AAS means two angles and a non-included side are equal. AAS would apply to our example if once again angles A and B are equal to angles D and E, respectively, and non-included sides, BC and EF are equal.
B E
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A /------------\ C D /--------------\ F