A preferred walk on is when they invite you to join the team without a scholarship.
A regular walk on is where your friend dies so you have a breakdown and go to a junior college next door to the school you want to go to for two years. Then you show up at practice and try to make the team. The coaches try to use you as a cheap motivational ploy for the good players, who all hate you. You get one lousy tackle in your entire career and they carry you off the field sarcastically. Then you embellish the story and they make a crappy movie about it. Then you get in trouble with the SEC for trying to inflate the stock price for your crappy sports drink company.