The Elven Ranger Corps was a high elven organization that encompassed the elven rangers of Quel'thalas, masters of marksmanship and scouting. It was charged with the defense of Quel'thalas and led by the ranger-general of Silvermoon, leader of the high elves' military forces. During the Third War, this organization was led by Sylvanas Windrunner. The Elven Ranger Corps met Arthas at the borders of Quel'thalas and fought the traitorous prince every step of the way, through both the Inner and Outer Elfgates, back towards the high elven capital.
The Elven Ranger Corps shares many similarities with the Farstriders, leading many to believe that they may actually be the same organization but with two different names. Another possibility is that the Farstriders was just one group that was part of the Elven Ranger Corps as a whole, as it is said that they are a small surgical strike squad and that "The Farstriders and other elven rangers" combated enemies.
However it is mostly likely the "Elven Ranger Corps" has simply been retconned to "Farstriders", as they have not been mentioned in more recent sources.