Unlike other software to program experiments, PsyToolkit uses scripts. There is no point-and-click graphical user interface. This has advantages and disadvantages. The disadvantage is that it takes somewhat more time to learn. The advantage is that it is ultimately much more flexible and much faster to programme an experiment than in other similar software.
This document gives a short intro to scripts, and the syntax documentation gives all the details.
PsyToolkit is a complex piece of software of many thousands of lines of code. It includes an online system that allows users to program and run experiments from within a browser, and since 2014 online data collection has become possible.
It is free of cost! This means that there are no cost barriers that keep people from designing and running cognitive psychological experiments.
How to learn PsyToolkit
It is best to start looking at examples and watching some of the videos. Once you understand the basics, you can use the syntax documentation to look up details.