To understand what isosceles triangles are: how to spot them, what their congruencies are, and how to find their angles.
This packet shows a video explaining isosceles triangles. It goes through typical isosceles problems, and is awesome in general.
Isosceles Triangle: a triangle with at least 2 equal sides and 2 equal angles
Base angles: the congruent angles of an isosceles triangle
Vertex angle: the third angle of an isosceles triangle formed by the legs, not congruent with the other two (unless its an equilateral triangle)
Legs: the two congruent sides of an isosceles triangle; opposite the base angles
Base: the third side of an isosceles triangle; forms base angles with the legs
This video explains isosceles triangles and how to find their angles.