Squares have 4 sides and they are all the same length.
Their edges or sides are straight
They also have 4 corners.
Rectangles also have 4 sides but they have 2 long sides and 2 shorter sides
Their edges or sides are straight
They also have 4 corners
Circles have curved edges and only 1 edge
They have no corners
Triangles have 3 sides or edges - tri means 3
They have 3 corners
The 3 sides can be different lengths