A bag you bring to school. It holds whatever you need for that day at school (besides, say, your violin case). It is a transport device cleverly designed to fit your needs in every grade.
In grade school middle school high school, the bag of choice is usually a backpack. Grade schoolers get tiny backpacks in pink and blue vinyl, maybe in the shape of a cartoonified animal, to carry six papers, a workbook, two pencils and a lunchbox. Middle schoolers (oh my god, I've stopped using the term "junior high", I seem to be from Michigan now) have slightly larger and plainer bags of heavy nylon canvas, in which they have to carry actual books and lunch in a brown bag. High schoolers tend to have serious backpacks, reinforced with leather and bearing an array of ribbons, buttons, writing in white-out, and iron-on patches; they carry books back and forth all day every day, switching them between classes, and either brown bag or buy their lunch.
College kids will usually already own this type of highschool bag, and may keep using it. It depends at least partially on what kind of shape the bag is in after (presumably) four years of hard use. They may find their bags to be falling apart or just plain too small; there are no lockers at college, so if you live too far away to go home between classes, you have to lug everything with you at once. So college kids have to buy new, bigger bags. They seem to pick backpacks over most other types of bags as well; they get the huge, heavy-duty ones with two large pockets, three or four side pockets, and reinforced shoulder straps (for less cutting action). Their keys dangle from a strap, by one of those aluminum mountain climbing clips; their water bottle dangles from the other. These bags are designed for the student who must have Everything on them at once, including their laptop. They are designed for constant travel, and minimal back pain (ha ha).
The other choice of college kids is the messenger bag. These seem like the tool of trendy kids to me; they come in an array of canvas styles and colors, with a prominent logo stitched to the flap. I have not personally owned one, but they seem flimsy and small; they seem like something you have to replace every year instead of every three or four. They are not an uncommon choice, but they just don't seem all that practical.
Then you get into grad school, and suddenly Everyone owns a messenger bag. Not a slick trendy-looking messenger bag like you find in undergrad, though; they are black or brown, and inclined toward being leather, depending on how much funding you have to work with. They are sleek and businesslike. They are a step or two from being an actual briefcase; some grad students do carry actual briefcases. These messenger bags are clearly designed more for show than substance. They are generally big and heavy enough to carry whatever books you need, but they only have one shoulder strap. This is not really a problem until you start trying to carry the thing around. Then, OW. You do not have a balanced load; your back is wrenched out of position. And grad students need the most books of all. Great!
I think grad students can afford this type of bag largely because they tend to have only one or two classes a day. Granted they are three hour classes which may require multiple books, but this load is manageable if you are not running all over the place every day. And grad students run around campus significantly less than undergrads. Their classes are all in the same department, after all, and often in the same building. Then they have offices in that department (ideally) in which to stash anything they don't need between classes: this is an incredible blessing. It eliminates carrying thirty five page papers around until you go home: you would otherwise have a full bag AND an armload. It lets you look impressive to the class you teach; you only bring the things you need (maybe not even your coat), and in a shiny professional leather bag to boot. Really, these bags and all their trappings are meant to look authoritative.
Schoolbags are very useful and happy tools. Think of how much junk you would have to juggle if you had no schoolbag, yet were trying to attend school. Think of how many papers you would potentially have flying everywhere if you just had to carry your books; think of what would happen if you dropped them. Take care of your schoolbag. It likes you.