The problem with having a bike on campus is that there is nowhere to store it during the winter. If you have a cheap bike, indoor storage could cost more than the bike itself. If you are planning on not riding much or at all during the winter and you have to store your bike outside, there are a few tips to keep the bike usable in the spring. 

If you do nothing, your bike will probably be unusable in the spring. Your chain will rust and at best will not work well. At worst, it will rust to the point of not working.  Your cassette or freewheel will also rust and may be destroyed by the elements.

While leaving your bike outside is not ideal, here are several tips that may help you avoid problems for very little money. 

Buy some bearing or general purpose waterproof grease. Tubes of grease can cost as little as a couple bucks. You don't want to use lightweight oil or lube as it will not last. Smear the grease over your chain and rear and front sprocket areas. Only do this if you are not riding the bike for the winter. If you ride it much, the grease will attract dirt and grime and reduce the life of your drive train. This will be messy and you will have to clean and degrease your bike in the spring, but at least you have a good possibility of your bike being usable in the spring. Also, put a coating on any stem bolts, brake bolts and your spokes, particularly on the nipple/spoke connection near the rims.  Take your seatpost out and lube it with the grease as well. A stuck seat post might destroy your bike.

Buy a bike cover. There are plastic bike covers that you can buy for $15 or less that will offer your bike some protection from the elements.  Not perfect but it probably will help keep some of the rain and snow off the bike.

Use shopping Bag for the seat. Tie a plastic bag around it to keep it dry while it's parked. It might keep the elements from destroying your seat, especially the UV rays. 

Apply Frame Saver or WD-40 for steel frames. You can use this spray to try to inhibit rust inside your bike for steel bicycles. While you are taking out the seat post to lube, spray some down your seat tube. Bicycle-specific frame saver is the best, but you can use a liberal amount of WD-40 in a pinch, or if you're on a restrictive budget.

Keep your tires inflated. To avoid damage, keep your tires inflated. You will probably only need to pump them up a few times during the winter. No matter what, your tires will wear more if they are subjected to the cold and light of the outdoors, but keeping air in them will minimize the damage.

Don't lock up next to the road before they plow the street or parking lot.  You may find your bike buried. Worse yet, they might damage your wheels or bike with the snow they push against it.

Don't forget a Spring tune-up. In the spring, your bike will need a good cleaning and a tune-up no matter what measures you take. They cost around 30 to 40 dollars, although the spring would be a good time to get a membership ($25 students and low-income, $40 for everyone else) at the Bike Project to help you work on your own bike. At a minimum your bike will need to be cleaned and lubed, including your drive train and all bearings.