CityBus is the main mode of transport around Purdue, seeing that they have a lot of Campus Loop services (Gold, Silver, Bronze, Ross Ade, South Campus) and a couple of others that run through campus (Tower Acres, Tippecanoe Mall, Salisbury, Purdue West/Klondike, Market Square, Wabash Trolley). If you really wanna get anywhere on time, either wait for the bus at least three hours early or get your own car.
The Salisbury bus is the bus that most students take to get to Walmart on Northwestern Avenue...and it only comes every two hours. So if you miss one, it's two hours to the next one. See what I mean? It helps to have the schedule for every bus that goes a certain somewhere, but they're not always on time.
You COULD fly in, but it'd be expensive. I think Northwest Metrolink is the only commercial line that flies into Purdue Airport. More than likely you'd fly into either Chicago or Indianapolis and drive. But you could also take Amtrak from either of those cities too. Greyhound has a stop as well.
Car and bus... and taxi too. That's it.