Frankly, I can't imagine anyone not having heard of this Alfred Hitchcock 
classic.  I find 'Psycho' to be Hitchcock's finest film for a plethora of 
reasons.

First a bit of advice.  There is no reason to have not seen this movie.  If you 
haven't, drop everything, and do so immediately.  Then read on.

'Psycho' executes the McGuffin better than any movie before or since.  (In 
retrospect, I think 'Malice' may have been attempting a Hitchcock-ian McGuffin 
with its serial killer subplot.  Regardless, it failed miserably.)  According to 
http://nextdch.mty.itesm.mx, "a McGuffin is a term for the device or plot 
element that catches the viewer's attention or drives the logic of the plot, 
especially in suspense films".  If you've seen the film, you know the "say 
WHAT!" moment I'm talking about.  If you haven't, suffice to say, be prepared 
for anything.  'Dusk 'til Dawn', a personal campy favorite, employs a similar 
device to wonderful results.

I don't want to ruin too much of the film while praising it, but the character 
of the villain is without reservation one of the best of the suspense genre.  A 
well developed character portrayed terrifyingly well.

What may make 'Psycho' a cut above other films of comparable quality is its 
modesty.  Most "great films" tell big or weighty stories, think great loves, 
great wars, devastating tragedies, and inspiring friendships.  Also, more true 
currently than historically, over the top camera work and editing try to vault a 
decent (or even good) movie to the point of greatness ('Traffic'?).  'Psycho' 
stands out from the normal great movie by telling a sinister but small story 
with incredible exactitude.  Hitchcock's direction provides an eerie backdrop 
for him to elicit stellar performances, but doesn't out shine his stars.  A 
great among greats.

This is absolutely one of my favorite movies.  Also making the cut are 'Bridge 
on the River Kwai' and 'Lovers on the Bridge', although neither are at all 
similar to 'Psycho' except in quality.