The best way to start learning about Desmos is to launch the calculator and just start playing with it. Make sure that you check out the Just Add Sliders, Tables of Data, and Regressions tutorials on the main page. These are some of the coolest features of Desmos.
Desmos sliders have transformed my teaching of linear functions. Sliders have helped my students understand how manipulating the slope and y-intercept of an equation changes the graph of the line. What are sliders? Check it out:
I also love the artwork on Desmos. Last year, almost half of my 8th grade honors students had used Desmos to figure out how to graph circles so that they could make their graphing art more interesting. One student was even graphing sine waves and animating them with sliders. He figured out how to do this in his spare time at home. In class, I never taught any graphing beyond linear functions. On their own, my students used the art examples on Desmos to learn so much more than that! Students can create an account so that they can log in and save their graphs.