What is a walkover in tennis?
A walkover happens before the first ball. If a player stops during a match, that's a retirement, and if a player is disqualified, it's a default.
Tour win-loss records generally don't count a walkover as a match won or lost, because no match was played. The draw simply shows the advancing player with "w/o".
In a match
A player withdraws with an injury the morning of their quarter-final. Their opponent reaches the semi-finals by walkover.
Questions
Does a walkover count as a win?
The player advances, but tour records generally don't count it as a match won.
What is the difference between a walkover and a retirement?
A walkover happens before the match starts. A retirement happens during it, and the match counts.