(Disclaimer: I learn and play the classical guitar.)
In the past, before the advent of high speed Internet connections, guitar lessons were restricted to tablature (tabs) published in ASCII format, or FAQs written in prose.
Youtube has revolutionalized the way people learn to play the guitar. Its free registration, and relatively lax copyright standards, have allowed amateur and professional guitarists to upload videos of themselves and masters playing. The compression technology ensures that video quality is not sacrificed for speed.
Unregistered users are also allowed to view these videos at no charge. Its robust search system (probably influenced by Google) is able to associate videos with similar contents together quickly.
Instead of following a score and attending classes, guitarists are able to now log on, compare the playing styles of Andre Segovia, Julian Bream, John Williams, and a host of other professional and amateur guitarists, and learn their fingerings (the ‘pause’ button is very handy for that). And all these are provided for free, with the exception of the broadband connection.
But there really is no substitute for the instant feedback that a good teacher can give.