TITLE: Reference for automorphic forms
QUESTION [12 upvotes]: I would like to know some reference to learn the theory of automorphic forms. Any (good) book or online lecture notes will be fine. I am particularly interested in the arithmetic point of view (e.g. galois representations associated).

REPLY [18 votes]: The theory of automorphic forms and its relationship to Galois representations is not something you will learn in one sitting, so to speak.  
For the very broadest outlines of the goals of the field, you might begin with
Mark Kisin's article What is ... a Galois representation.  For a discussion of one of the fundamental recent advances in the field (which is 
nevertheless rather out of date, since the field is moving very rapidly at the
moment) you could read Barry Mazur's article on the Sato--Tate conjecture.
To actually learn the theory, I recommend a solid knowledge of the theory of modular forms, which is a part of the theory of automorphic forms for the particular group $GL_2$ over $\mathbb Q$.  You can start with Serre's Course in Arithmetic; after this, Shimura's book mentioned in amWhy's answer is a possible subsequent text, or there is Bump's book mentioned by Amitesh.  Both of these still only deal with the group $GL_2$ over $\mathbb Q$, though.
For automorphic forms on general groups, the standard way to learn the outlines
of the field still seems to be to read the Corvalis volumes, available
here.  These are not easy reading, and I don't recommend studying them in isolation --- you will need a good advisor (or equivalent) to guide you through them and relate them to
modern developments and concerns in the field.
Unfortunately there are no particular modern texts that deal with what you are asking about, although there have been many recent conferences since the proof of Sato--Tate which have online videos, e.g. at MSRI in 2006 and at the CIRM in Luminy in 2007.  You could try watching some of these for more modern perspectives.  
There is also the recent article The conjectural connections between automorphic representations and Galois representations by Kevin Buzzard and Toby Gee.  This is written for experts, but the introduction and bibliography may give you some additional hints for reading.