This question originally appeared on Quora. 
Answer by Dan Levin, Commercial pilot and COO of Box, Inc.
Air is like water. As it moves over a rough surface, just like a stream flowing over a rocky bottom, it gets stirred up. So any time that we have fast moving air (i.e. high wind speed) over a bumpy area (i.e. mountains), we are going to have the possibility of turbulence.
Also like water, if we have two streams of air hitting each other, we are going to have turbulence. Imagine a tributary flowing into another river - at the intersection the water is rough. So if we have two moving air masses, or two weather fronts, that are running into each other - we are likely to have turbulence.
In addition, any time that we have a major weather event - especially a thunder storm or other large weather event where we have air moving up and down (that is what a thunder storm is, basically), then we are going to have turbulence around that storm.
Clouds are also the result of air rising until the water vapor in them condenses (massive simplification, of course), and so clouds are sometimes turbulent (especially if they are cumulus clouds - the kind that look like cotton puffs).
Finally, if we have two layers of air that are moving at different speeds, we are likely to have turbulence at the layer junction.
All of this is predicted by a couple of weather products produced for pilots by the NOAA. You can see them too. The turbulence warnings, called AIRMET's, can be seen at ADDS - Turbulence .
At that same page, you can also view the Graphical Turbulence Guidance product, which gives very good guidance about the likelihood of turbulence in the continental US.