Sure. Also, be aware that you can search threads here by topic and this is a perfect example of a topic that has many threads that explain.
Short version - A true walk-on is someone who shows up at the fall try-out that schools have with no previous agreement or arrangement with the coaching staff - basically trying out for the team with little or no introduction.
From there, it becomes case by case. With a preferred or recruited walk-on, the definition is what the coach states it to be but nothing is binding. Sometimes, a coach will recruit a kid but not willing or able to offer scholarship money for any of a variety of reasons, so offers a Spring roster spot. Sometimes, he offers full participation thru Fall with assurance that he will get every opportunity to be seen for that duration and try to make the team. Sometimes, it is just assurance that he will get a good look in the fall. Aside from a guaranteed Spring roster spot, each of these other scenarios puts the player in an unlikely position to make the team and get playing time. He is behind all other players who are either on athletic scholly or guaranteed a roster spot. All things being equal, he loses out when rosters are finalized. In the process, he is less likely to get as much serious attention, instruction and rep opportunities than the others.