Almost everyone has seen paper airplanes soaring through the school yard (or in the classroom in some cases) or beautiful paper cranes lining the walls of a room. These are common examples of origami that can be seen in daily life. Through the use of a single sheet of paper orgamists are capable of producing simple and fun models as those of Jeremy Shafer or complex designs such as those in The Works of Satoshi Kamiya by Satoshi Kamiya.
In simple terms, origami is simply folding a sheet of paper such that it produces an end result that you desire. A variety of different types of folds are often described when speaking of origami: inside reverse folds, outside reverse folds, rabbit ears, open sinks, closed sinks, squash fold, crimps, pleats, and the list goes on. However, all these folds are simply different combinations of two simple folds, that are, in a sense, still the same type of fold. These two types of folds are the mountain fold and the valley fold.
The mountain fold is when one has a sheet of paper and folds it in such a manner that the fold forms a mountainous shape such as that of a carrot when typing: ^. If the paper to be lain on a flat surface, the two sides of the fold would be folded in the direction into the flat surface. This would be often done by holding the paper up and folding the paper underneath. In a diagram, printed (or digital in the digital age) instructions of how to fold a particular origami model, mountain folds are often depicted as a dot - dash line.
The valley fold is simply the opposite of a mountain fold. The paper is folded upwards so that the overall shape of the crease is that of a valley or "V" shape. When lain on a flat surface, one side of the fold can simple be lifted up to form the valley fold. In an origami diagram, the valley fold is often represented as a dashed line.
As can be seen from these two description and pictures, the mountain and valley folds are not that different and can be interchangeable. All one has to do is flip the paper around to make a mountain fold into a valley fold or a valley fold into a mountain fold. However the distinction is still necessary in diagrams to see the orientation of every fold. With just these simple folds, a multitude of different models can be created.
I have folded origami since preschool and suggest that you try it out yourself. Although it requires a lot of patience, the end result will surely leave you satisfied.