What is the difference between def foo = {} and def foo() = {} in Scala?

Question

Given the following constructs for defining a function in Scala, can you explain what the difference is, and what the implications will be?

def foo = {}

vs.

def foo() = {}

Update

Thanks for the quick responses. These are great. The only question that remains for me is:

If I omit the parenthesis, is there still a way to pass the function around? This is what I get in the repl:

scala> def foo = {}
foo: Unit

scala> def baz() = {}
baz: ()Unit

scala> def test(arg: () => Unit) = { arg }
test: (arg: () => Unit)() => Unit

scala> test(foo)
<console>:10: error: type mismatch;
 found   : Unit
 required: () => Unit
              test(foo)
                   ^

scala> test(baz)
res1: () => Unit = <function0>

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Update 2012-09-14

Here are some similar questions I noticed:

1. Difference between function with parentheses and without
2. Scala methods with no arguments