Congruent shapes: shapes which are identical and are the same size. This is even true if the shapes have been rotated or reflected. The shapes below are congruent.
When two shapes are the same but different sizes, they are said to be similar. The triangles below are said to be similar.
To find a length of a similar shape, you want to find out what the scale factor is (how much bigger is the larger shape). Then multiply the length you do know, by the scale factor.