On this page you can see the explanation of the symbols I am using in my diagrams. They are based on the widely (practically exclusively) used Yoshizawa-Randlett system. Some of them (for example the "fold and unfold" arrow) are used differently, but most origami diagrams can be understood knowing these symbols.
Some modifications and additions originate from the American folder John Montroll. Such are the modified forms of "fold and unfold" and "pull out" arrows, as well as the "push here" and "put your finger here" symbols. These are used by several others and are well known by the origami community. A minor addition that I would like to propose here is the usage of "fold over x layers" arrow, which can be very helpful in cases where only some of the many available layers should be grabbed.
Concerning the mountain fold line style: there are two forms in common use. Some people like the one-dot version, while others prefer the two-dot version. I belong to the latter camp, but it is good to know about the other option, as you are guaranteed to see it in different diagrams.
The diagrams below are free to download for personal use only.
In all other cases my permission is required.