Over the years, I have met many guitar students who could not actually
play anything, even though they had taken lessons, perhaps for years.
If they were asked to play something, the best they could do would be
offer you some isolated "pieces" of songs or solos they had worked on.
They are missing certain pieces of knowledge about the Art & Science of
practicing that would enable them to get past a hurdle that stops the
progress of many guitar students: how to actually finish something you
are learning, and bring it up to what is called "performance level",
which is another way of saying "I can get through this without it
falling apart so badly I can't keep it going", or "I play it all the
way through well enough to not cause me major embarrassment, or the
listener major discomfort, or pain".
Read the entire lesson here:
http://www.guitargearheads.com/modules/soapbox/article.php?articleID=33