They're synonyms in the sense that they both mean to study hard. The difference is the time-frame: 'swot' means to study hard (verb) and a swot (noun) is someone who studies hard. To 'cram' is to study intensely over a short time, e.g. if you have an exam on Monday you would cram over the weekend (literally cram or force the information into your mind). A 'crammer' is slang for a school or institution that prepares pupils for exams in a short period of time,

To swot is to study. Cramming is when you do the swotting at the last minute...e.g. the night before.

Well... I can tell you that they aren't synonyms to me because "swot" is a word that I haven't heard before. I'm seeing that it's an informal British word so maybe somebody from over there can answer.