What is Yarl?
1.
Yarling is the vocalization process of putting a hidden 'R' underneath every syllable in a lame way to sound soulful. Examples of bands that regularly yarl in their vocals; Matchbox 20, Nickleback, Pearl Jam
"Eddie Vedder is a yarler"
"Rob Thomas doesn't sing, he just yarls"
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2.
To sing like Eddie Vedder from Pearl Jam. People like Scott Stapp (Creed), Jason Wade (Lifehouse), Alex Band (The Calling), are known for having Vedder-like voices. Another person known for yarling would be Scott Weiland (Stone Temple Pilots, Velvet Revolver), but after he was trashed for yarling like Vedder, he gradually began to sing differently and now sounds nothing like Vedder.
Although it has been said that Eddie Vedder's yarling was taken from people such as Ray Charles and Bruce Springsteen, the similarity between Vedder, Charles, and Springsteen is nowhere near as close as Vedder, Stapp, Wade, and Band.
"God, Scott Stapp is such an unoriginal bastard. He yarls exactly like Eddie Vedder."
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3.
Typically, it's solely dependent on the situation it's used in.
Essentially, it has the same basic meaning as 'eh'; 'meh'.
The actual defenition is usually deemed as whatever the hell the author (speaker) feels compelled to make it mean.
Person A: "Dude, let's go see a movie"
Person B: "Yarl, I got shit to do.."