The Valley fold is represented by a dashed line and an arrow with black tip. It's called "valley" because the fold reminds a valley.

The Mountain fold is represented by a dot-dashed line. It can either be one dash and two dots or one dot only. The arrow has a white tip. The fold reminds a mountain.

The edges of the paper are represented by a black solid line. The crease lines are lighter or more narrow. The X-ray lines show parts of paper that are hidden behing other parts and are represented by a dotted line.

The Fold and Unfold arrow indicates that you must make a crease where it says to. The Pull out arrow indicates that you must pull out a certain part of the paper that is located inside the model.

Both arrows means the same: the paper will be enlarged so that it will be easier to visualize the next steps. Sometimes only a part of the model is enlarged then this part is circled, and this circle is enlarged.

The arrow on the left indicates that the model must be turned over. The arrows on the right indicate that the model must be rotated. The number inside the arrows indicate how many degrees it must be rotated.

Sinks are folds where you have to push in the paper. There's not much to say about the difference between Open and Closed Sinks. Look at the detail of the folds, both show how it should like inside. In the Closed Sink you have to lock the layers and in the Open Sink you keep them free.

The Reverse Folds are done in points of the paper where there's two layers of paper like in the pictures on the right. In both Inside and Outside we have to reverse the folds of the tip of the paper. For example: a mountain fold becomes a valley fold. That's why it's called "Reverse" fold. It can be either done to the inside or to the outside.

The Pleat Fold, also called Stair Fold, is a fold composed by valleys and mountains alternately. The example on the left shows a Pleat Fold composed by one Valley and one Mountain.

These symbols indicate in how many equal parts the paper must be divided. It can be done either with an area or with an angle. It's very impotant when you want to fold accurately.

There are two ways to indicate that you must repeat a certain number of folds. One is by putting the arrow on the left, the number of times it's crossed indicates how many times you must repeat a fold. You may also find the box at the right of the pic, inside the box there are the steps you must repeat.