To keep from becoming too much of a bore, this will be a series of posts covering the different components of a camera system and I will be able to go into more depth with each post. I need to caveat all of that by saying that I'm not able to build a camera or component from scratch but I can pick out the right gear for my needs based off of my own personal research and experience. I haven't always been like this but I still heed some of the information given to me from my old buddy, Ed.
Let's just assume that everyone reading this is looking into purchasing a DSLR and they know what DSLR stands for. If you don't know what you are looking for, here is a link to a short video from a source I reference from time to time for information about the different kinds of cameras and their potential uses.
The first thing I want to address is what brand camera to get. That question is just as loaded as asking which car manufacturer is better; it really depends on who you ask and maybe what your uses for it are. Canon, Nikon, Sony, along with several other brands, all have their pros and cons and if you were to call me up and ask me for help, I would tell you that I can only tell you about Canon because that is the only brand I have ever used. The reason I tell people that is I assume that if they are asking me for purchasing help, chances are they are going to ask me how to operate it in some way. I don't mind helping but I want to be able to help you and to do that it would just be easier if you bought a Canon; sound familiar, ED? LOL But, I do tell them to do their research and in most cases they have bought a camera other than Canon.
Find a camera in a price range you are comfortable with spending money for. DSLRs run anywhere from about $500-$7,000 just for the body. You are also going to have to buy a lens, memory card, and some sort of camera bag for transportation and storage. When it comes to DSLRs there are basically two main types, full frame (FF) and crop sensor (APS-C). A full frame is the same size as a 35mm slide of film but a crop sensor isn't and has a multiplication factor to equal 35mm. I know for Canon, their crop factor is 1.6 but I believe Nikon is 1.5. That becomes an important number when it comes to lenses later down the road. A full frame sensor will typically handle much better in low light situations. The APS-C sensors have come a long way with low light since I've been shooting but a full frame should usually still out perform the crop.
FF cameras are widely used in wedding and portrait photography. Now that's not to say that a crop camera can't be used for those but this is what my research has shown me. A crop camera is usually used in sports or wildlife photography due to the crop factor to get extra reach. That is also not to say that FF camera couldn't be used for those. The technology has come so far and so fast that honestly it's the other bells and whistles of a camera that will help you decide what you are going to purchase for your application.
Some of the things you will want to look for in the specs and reviews are things like focus points (how many and what type), shooting modes, ISO range, how the auto focus (AF) works, and maybe frame per second (FPS). If you plan on doing video you will also want to look up the video specs a camera has as well as maybe an articulating and/or a touchscreen LCD. There are a lot of bells and whistles when it comes to cameras but those are the types of things I look for. I bounce this information off of reviews, not from one source, and I compare pictures in the styles I plan on using them in. Something Ed told me is not to put too much credence in reviews because people that bitch will always find a reason to bitch and people that do will find a way to do. It is still important to look at other people's experiences so you can ultimately decide for yourself if you are comfortable with a camera that lacks in a particular area.
I know this isn't an all encompassing or extremely detailed guide, but it should point you in the right direction or at least get you to asking the right questions when you are looking. There are hundreds of camera bodies with different configurations by different manufacturers. In the end, it is what you are comfortable with spending and features you can do with/without that will guide you to your camera. One valuable lesson that I learned in my time is that I quickly outgrew my first DSLR but my next one was my primary for the next six years and I still use it today as a second/backup camera. Even though the technology on it is dated, I can still take great pictures with it and I am very proud of that.
If you have any questions, leave a comment and I will try to answer it for you, and at a minimum, point you in the right direction for help.