This blog article "Deepfakes: What They Are, Why They Matter", describes deepfakes as
"...a technique in which artificial intelligence-based technology is used to alter or produce video content. Essentially, a deepfake is a video of something that looks like it occurred, but truly did not."
If you really want to dig deeper, see these articles:
Farid, Hany. "How to Detect Faked Photos" American Scientist 105 (2) p.77, 2017.
Wen, Tiffanie. "The hidden signs that can reveal a fake photo" BBC Future,