Scala

Type bounds

B <: U imposes a upper bound on type U, while B >: L imposes a lower bound. This is useful for generic methods that takes only classes on a particular inheritance tree as its type parameters.

The official tour has a pretty good explanation: Upper bounds and Lower bounds

Self Type

A trait with a self type cannot be used without mixed in that trait first.

trait User {
  def username: String
}

trait Tweeter {
  this: User =>  // reassign this
  def tweet(tweetText: String) = println(s"$username: $tweetText")
}

A question is how it is different from trait subclasses.

See Official tour, a discussion on the cake pattern and a StackOverflow question about this.