Difference between fold and foldLeft or foldRight?

Short answer:

foldRight associates to the right. I.e. elements will be accumulated in right-to-left order:

List(a,b,c).foldRight(z)(f) = f(a, f(b, f(c, z)))

foldLeft associates to the left. I.e. an accumulator will be initialized and elements will be added to the accumulator in left-to-right order:

List(a,b,c).foldLeft(z)(f) = f(f(f(z, a), b), c)

fold is associative in that the order in which the elements are added together is not defined. I.e. the arguments to fold form a monoid.

fold, contrary to foldRight and foldLeft, does not offer any guarantee about the order in which the elements of the collection will be processed. You'll probably want to use fold, with its more constrained signature, with parallel collections, where the lack of guaranteed processing order helps the parallel collection implements folding in a parallel way. The reason for changing the signature is similar: with the additional constraints, it's easier to make a parallel fold.

You're right about the old version of Scala being a problem. If you look at the scaladoc page for Scala 2.8.1, you'll see no fold defined there (which is consistent with your error message). Apparently, fold was introduced in Scala 2.9.

For your particular example you would code it the same way you would with foldLeft.

val ns = List(1, 2, 3, 4)
val s0 = ns.foldLeft (0) (_+_) //10
val s1 = ns.fold (0) (_+_) //10
assert(s0 == s1)