Terminology - Valley and Mountain Folds

Every origami starts off with either a Valley or a Mountain Fold.

A Valley Fold is a fold where the paper is folded into itself.  When unfold, the paper will form a "V" shape.

Here is an illustration:

Pic 1:  A paper folded diagonally into itself to get Pic 2

Pic 2:  Valley Fold

Pic 3:  Valley Fold as a result of paper being folded horizontally

With a Mountain Fold, the crease is folded away from itself to form a peak of a mountain.

Illustration as follow:

Pic 4:  A paper is folded diagonally away from itself to get Pic 5

Pic 5:  Mountain Fold

Pic 6:  Mountain Fold as a result from paper being folded horizontally

If we flip over a Mountain fold, we will get a Valley fold. For instance, flip over Pic 6, we get the "V" shape, and that itself is a Valley fold.

The difference between these two folds may not seem significant.  However, it will sure give an impact on how your folding turns out.  In most cases, the correct fold will help the paper to "fall into place" naturally.

To avoid confusion, I will indicate the side of the paper on which a mountain or a valley fold will be applied.