scala: 'def foo = {1}' vs 'def foo {1}'

Question

what is going on in each of these forms of defining foo?:

scala> def foo = {1}
foo: Int

scala> foo
res2: Int = 1

But:

Answer 1

found this in http://anyall.org/scalacheat/:

//[bad!] 
def f(x: Int) { x*x } //hidden error: without = it's a Unit-returning proc; causes havoc