Hello, thank you for your question!
The square is a geometric figure that has 4 equal sides. Here is an image:

The diamond is a geometric figure which also has 4 sides! There are two pairs of equal sides. Also, there are 2 obtuse angles and 2 acute angles (two large angles and two smaller angles). Here is an image:

For the difference between the diamond and the square, these are the angles that you have to look at! Are they the same and straight (like the corner of a sheet of paper) or not?
The rectangle is a geometric figure which has 4 sides, which are although not all equal. There are two pairs of equal sides. Two sides are therefore longer than the others. For the angles, they are the same as the square. Here is an image:

The triangle is a geometric figure that has 3 sides. Here is an image:

The circle is a round geometric figure, with only one side. Here's a picture:

Don't hesitate if you have more questions! :)
Good evening,
Audrey