Places that are crowded are packed with people. During rush hour, you might decide not to get onto a crowded subway car, but wait for the next one, hoping it will be less crowded.
Rock concerts are often crowded, and sometimes classrooms are so crowded with students that it makes the teacher's job harder. Tokyo is a crowded city, especially compared to Dubuque, Iowa, and shopping malls generally become crowded on the weekend before Christmas. These places are all crowded with people, but you could also say that a field is crowded with cows or your friend's house is crowded with cats. The Old English root is crudan, "to press."