A Red Shirt Freshman is a student on an NCAA college sports team who was kept out of competition for a year in order to prolong his or her eligibility. Usually this is done to allow the student to be a "part" of the team in terms of learning, healing from injury, and/or conditioning, but not a participant in any of the contests/games/meets/etc.
The following year, they begin their 1st of 4 years of eligibility, or their "freshman" year of eligibility, even if they are sophomores in terms of academics. (urban dictionary.com)
Walk on -- player that is not on scholarship
True Freshman -- player who plays in games and has not used a redshirt.