Defining what "sleeper" means is one of my least favorite Fantasy activities. It's right behind arguing with someone over what makes a sleeper. Boring. To spice it up, let's use the New York Jets and their collection of sleepers to highlight the different types.

Matt Forte is a bad running back. At least he was last season. He's also 31 years old and got hurt in 2016. Why would anyone want to draft him? Well, he had 248 touches and finished as the No. 20 RB in just 14 games. He's falling into the double-digit rounds in drafts. Matt Forte is the "twist my arm" sleeper.

Bilal Powell has never rushed for more than 722 yards in a season. He's opened the year as a backup for six seasons in a row. But, man, did he look good for four games last year. Powell had over 500 yards and three TDs in his final four games of 2016. Bilal Powell is the "what if the coach figures out he's better than the starter?" sleeper.

Quincy Enunwa scoffs at the bad quarterback excuses you've made for Brandon Marshall. On the same team, with 23 fewer targets, Enunwa had 69 more yards and one more touchdown. His floor is No. 4 WR production, but he'll see more targets with Marshall and Decker gone. Quincy Enunwa is the "targets over everything (TOE)" sleeper.

ArDarius Stewart is an exceptional athlete, and the receivers in front of him aren't that great. He'll play with bad quarterbacks who need receivers like him who make plays after the catch. You can draft him at the same time other people are drafting defenses and kickers. ArDarius Stewart (and Robby Anderson for that matter) is the "he's young, he's cheap, and we haven't seen him fail" sleeper. 

Those aren't the only types of sleepers, but you'll find bits and pieces in these 10 sleepers as well:

Cameron Brate Tampa Bay TE
Yes, I know Tampa Bay drafted O.J. Howard. No, I don't believe that Brate is a better talent. But we've seen what often happens to rookie tight ends, and it's highly unlikely that Brate just disappears from the game plan. You don't need to draft him in a standard 12-team, 15-round draft, but I'm very fond of him in deeper drafts and best-ball leagues at a cost of almost nothing. He'll should be a good streamer early in the year.
Like Brate, Matthews saw his team draft his replacement. The difference is that while Corey Davis is only going to replace Matthews as a No. 1 WR, Matthews will still be a starter and have a huge role in the offense. I don't even think he sees a reduction in the 108 targets he saw last year. While his touchdown rate will likely decrease, Matthews still looks like a low-end No. 3 in Fantasy and is going to be pushed way down draft boards because of Davis' arrival.  
Vance McDonald San Francisco TE
Some guys you just can't quit, and Vance McDonald is one of those guys for me. This is a team that is low on quality options in the passing game and will be playing from behind all season. There were rumors in the spring that the 49ers were shopping him, but I don't think there are too many pieces they're holding on to tightly right now. This is a full rebuild. Still, there are targets to go around, and McDonald has the talent to perform like a borderline No. 1 TE. 
DeSean Jackson Tampa Bay WR
In case those first three were a little too deep for you, DeSean Jackson has been a steal in drafts I've done so far this year. He's consistently falling to Round 7 or later. Jackson's upside is hurt by the fact that he's paired with Mike Evans, but he still profiles as a low-end No. 2 WR in Fantasy. He hasn't had more than 100 targets since 2013, but he has still put up a pair of 1,000 yard seasons. Jackson's big-play ability in this offense will lead to some huge weeks, making him an excellent best-ball selection.
C.J. Prosise Seattle RB
The Seahawks brought in Eddie Lacy and have Thomas Rawls, but Prosise's role in the passing game should be secure. He flashed in 2016 and opened up a new dimension in the Seahawks offense. Even if Lacy and Rawls stay in shape and healthy, I see a path to 50 receptions for Prosise. If either or both get hurt, he has enormous potential. Prosise brings something to the table that neither Lacy or Rawls provides. He just has to stay healthy.
Josh Reynolds L.A. Rams WR
Reynolds was drafted by the Rams, which is both a good thing and a bad thing. Jared Goff doesn't exactly inspire confidence, but neither does anyone currently ahead of Reynolds on the depth chart. What I love most about Reynolds is his ability to win in the end zone and provide big plays. Those are two things that can lead to Fantasy points even if targets are limited. Reynolds is available in the last round of most drafts and has No. 3 WR potential this year.
I could easily put DeAndre Washington's name here as well. Both Raiders running backs were efficient in 2016 and occupy an enviable spot behind a 31-year-old running back who I believe will be a bust. I lean towards Richard because of his big-play ability and the fact the team used him more in the passing game last year. That role will be open regardless of how Marshawn Lynch performs. 
Stafford has thrown for at least 4,200 yards each of the past six seasons, and it doesn't seem like anyone gives him a chance to be a No. 1 quarterback this year. Part of that is because of the depth of the position, but I'd guess a bigger part was the fact he only threw 24 touchdown passes last year. I'm expecting Eric Ebron to take a step forward, which should help Stafford in the red zone. Stafford won't get back to the 32 he threw in 2015, but I would definitely expect more than 24.
Mike Wallace Baltimore WR
Maybe my two favorite sleepers play for the Baltimore Ravens, and there's a good reason why. The team lost Steve Smith, Kamar Aiken, Dennis Pitta and Kyle Juszczyk this offseason. That's 321 targets they have to replace. The addition of Jeremy Maclin will take care of some of that, but Wallace was a 1,000 yard receiver last season with 116 targets. He should see a 10-15 percent increase in targets, which will make him a borderline No. 2 option. 
Danny Woodhead Baltimore RB
Another piece the Ravens added to fill that target void was Danny Woodhead. Woodhead has missed a big chunk of two seasons in his past four, but the two seasons he stayed healthy he was one of the best pass-catching backs in football. If he plays 16 games, I'd expect him to be at or near the top of the target rankings for all running backs. Woodhead's real value will be in PPR leagues, but he gets almost completely ignored in non-PPR, which makes him a good value in both formats.