I didn't mean to make it sound like I actually understand stuff.
None of this is easy, and I'm sure I had to read my relativity book a few times before I grasped the small portion of it that I did.
My short post isn't complete - it really is only 2c worth. So, for example, to be more strictly correct, even though a photon is definitely a massless particle, and has zero mass "in the real world", in Relativity, it has relativistic mass and momentum!
More here.