Sliders have been mentioned on many of the other pages here, simply draw a graph of y=mx+c for example and Desmos will ask if you would like to add sliders. It is also possible to set up an automatic animation using your chosen speed.
Sliders can also be used in other sophisticated ways, with a domain for example.
Using a slider within a domain makes it possible to show the curve is traced out, particularly useful for polar graphs but also for any curves. (There are some useful resources on polar curves here for students including some further Desmos pages).
Experimenting with a quadratic graph, I have plotted some points and can then draw the curve through these points.