Wandering Crows is the nickname received by sworn brothers of the Night's Watch who travel through the Seven Kingdoms recruiting new members for the order. Usually, these recruits are lowly criminals - rapers, thieves or murderers - that have chosen to take the black to avoid execution or mutilation. Thus, the few Wandering Crows typically scour the dungeons of the five major cities of the Seven Kingdoms (such as King's Landing itself) and several of the larger towns.
But there are also volunteers, for example poor boys and men, searching for a opportunity of regular food and shelter, and also highborn sons on the search for glory and honor.
The Night's Watch consists of three branches: the Rangers who form its core fighting force, the Builders who maintain their structures and the Wall itself, and the Stewards who produce their day to day supplies. Wandering Crows are few in number, and are not technically considered a separate branch of the Night's Watch. Instead, recruitment duty is carried out by trusted members drawn from the other branches. Sometimes this is a temporary assignment, though often those who can no longer fight settle into it as a permanent position.
Recruiters are the only members of the Night's Watch who have regular leave to travel away from the Wall, as they pass through the southern kingdoms carrying out their duties. Unlike most other members of the Watch, they therefore tend to maintain a few more contacts with the outside world. Given that travel to any of the southern kingdoms from the Wall must pass along the Kingsroad, and therefore past Winterfell, they tend to be fairly familiar with the castle. This also means that recruiters such as Yoren feel a certain loyalty to House Stark which pushes the bounds of their vows of strict neutrality in political affairs.
While sometimes other members of the Night's Watch will fill a temporary tour as a recruiter, other recruiters like Yoren settled into the position permanently due to injuries which prevent them from serving as frontline soldiers, but still physically able to travel long distances. Yoren became a recruiter after a bad shoulder injury meant that he could never fight well again. When Arya encounters him, Yoren has been serving as a recruiter for thirty years, and in all that time has lost only three recruits on the way to the Wall.