If a place, a bag, or a room is crammed, it is so full of things or people that almost nothing else can go in. Bodies are pressed together on a train; clothes are pushed into a case; a drawer will not shut. It feels tight, stuffed, a bit uncomfortable — more than “quite full.” You can cram things into a space (that is the verb), and then the space is crammed (the -ed form, often describing a state: it is already packed). It is ordinary, slightly vivid spoken English. Crowded is the everyday word when we mean too many people; crammed can be people or things, and it sounds more “forced in.” Though actually, the same verb is the one students already know from exams: to cram is also to push facts into your head in a hurry. When we call a bus crammed, we mean the packed-full sense, not the studying sense — but you can hear the same picture in both: pushing more in than fits easily.