var declares a mutable cell. Analogous to a non-final in Java. Avoid the use of var unless in exceptional circumstances.

val declares an immutable cell that is evaluated at its declaration point. Analogous to a final in Java.

def declares an immutable cell that is evaluated upon each use. An argument list may follow. Analogous to a method in Java.

lazy val declares an immutable cell that is unevaluated until first use.

Example (welcome to the Scala interpreter)

scala> val a = 7
a: Int = 7

scala> def b = 8
b: Int

scala> def c(n: Int) = n + 42
c: (Int)Int

scala> var d = 9
d: Int = 9

scala> lazy val e = 10
e: Int = 10