Shapes

I have four sides, all the same exact length....what shape am I?

Square

I am perfectly round, with no corners...what shape and I?

Circle or Oval

I have five sides, all the same length...what shape and I?

Pentagon

I have three sides, and all my sides do not have to be the same length...what shape and I?

Triangle

I have four sides, each side opposite the other is the same length...what shape am I?

Rectangle