Transforation geometry -- using matrices!

This is a question from GCSE level, so please offer a simple explanation if possible. :)

I have been given the following:

To rotate a point (x,y) 90deg anti-clockwise, pre-multiply the x y matrix with:

Similarly, for a an anti-clockwise rotation of 180deg, it's:

p.s., how can I create a matrix in this forum in the math tag?

It's tough to remember so memorize so many matrices when there are very similar looking matrices for reflection (on x axis, y axis and y = -x). I think perhaps, an explanation of how these matrices are created will help me. Or maybe some pointers on how to memorize at least 8-9 such matrices.

Thanks ...