Difference between PSNR and SSIM comparison

Hello everyone.

I have been reading about this but couldn't understand properly that's why posting it here. How would we differentiate between PSNR and SSIM comparison of two images? I mean when we compare two images, what are we really looking into via PSNR comparison and what are we looking into via SSIM comparison? Also, how is SNR different from PSNR? Kindly explain in the context of images or 1D signals.

Thanks in advance.