trait is like a Java interface but allows mixin composition.

scala> trait Foo { def bar(n: Int): String }
defined trait Foo

scala> trait Bar
defined trait Bar

scala> trait Baz extends Foo with Bar { val baz: java.util.LinkedList[Int] }
defined trait Baz
              

class is similar to a Java class, however case class has no Java equivalent.

scala> case class Person(name: String, age: Int)
defined class Person

scala> val p = Person("Bob", 42)
p: Person = Person(Bob,42)

scala> val z = p.name
z: String = Bob
              

A sealed class or trait restricts possible subtypes to the declaring source file. Java has an obtuse equivalent by exploiting the fact that only nested classes may subclass a class with an inaccessible constructor.

sealed types and case class are used to denote Algebraic Data Types.