There are a number of types of patent applications, and a couple types of patents, but when we speak of a "patent application," generally we are talking about a utility patent application -- aka, regular patent application (RPA).  This application is your formal request to the US government that it protect your constitutional rights to your invention, which are conferred on you by Article I, Section 8 of the U.S. Constitution.  If the PTO decides that you have an invention that is protectable under the Constitution, it will issue Letters Patent to you, which is what you march into court with if someone attempts to steal your invention.  

The RPA consists of a legal disclosure (including drawings) and a detailed claim of your invention.  You are required to disclose to the world how to "make and use" your invention.  This is your part of the bargain with the government.  In return for your disclosure the government will grant you a monopoly for [now] 20 years from the date you file.  But your monopoly will be strictly limited to just what you claim in the RPA.  One's foot looms as a large target for those who do not understand this important point.  

Your "claim" is actually a set of numbered claims that set forth in detailed language all of the elements and limitations of your invention.  If you have never seen or read a patent, below is a link to a very simple patent of my own that will give you a good idea of what a patent is.  This patent gives me a monopoly to make, use and sell a lever for bar taps that allows the barmaid to fill a glass or jug using just one hand.  [As you can imagine, the research for this project was horrendus.]  This patent is displayed publicly on the PTO's web site, which is where yours will be displayed once it is issued.  

Your RPA is very important because once it is allowed by the Patent Office, it actually becomes your patent.  What you claim in the application as your invention is all you will be entitled to protect in court.  The RUPA is, thus, the "title" to your invention, and like a real estate title it must be drafted carefully in order to comply with currently applicable laws.  It is quite possible for you to draft and file your own RPA and obtain a patent on your own if you have enough time and incentive.  In fact, the Patent Office is even required to assist those who apply for their own patents.  A DIY book from Nolo Press by David Pressman (Patent It Yourself) takes you through the process step-by-step.  Even if you hire a professional, this is a good book to have at hand.  

But be forewarned: all patents are not equal, and some patents are not worth the effort it took to print them.  To extend our analogy with real estate, when people ask me whether they can draft and prosecute their own patent, my rule of thumb is that anyone who would not be willing to do the legal work required to buy a new house --  draft the contract, draft the deed and title, and record the deed -- on their own probably wouldn't want to pursue a patent on their own either, because drafting a good, strong patent is, in most cases, much, much more difficult than drafting and recording a deed and title to real estate.   That is why inventor's hire patent professionals.