It's a series of textbooks [1] based on lectures Feynman gave while teaching the introductory physics sequence at Caltech.
To my knowledge, no universities actually use the Feynman Lectures as a textbook for their introductory courses. However, some students who have already taken physics read the Feynman Lectures later and (self-)report a much clearer understanding.
FYI, the audio of the lectures is also available, but can be difficult to find.
Afair, some of its a little scrappy at the start (poor recording setup?) but his delivery is great. I wish there was video available.
