After playing around a bit with camera tracking, I found that you need to know some key information about your camera that you're using. One bit of information that kind of confused me, was the sensor size.
Using my Kodak Zi8, for camera tracking I was confused by the 1/2.5" sensor size. So I took some time to understand this concept.
First thing to note, like TV sizes, sensors come in a size "type" not "exact size". So my 55" inch TV is really about 54.5"... and who knows the exact size of my sensor in the Kodak Zi8? Well, 1/2.5" is about good enough for what I'm doing, but it's good to know.
But blender doesn't work well with the 1/2.5" formatting of a sensor size for now, it asks us what is the mm width. Luckily wikipedia has information about camera sensor size formats.
So when you're working with a still image, or the camera track builder, this is a key item to know. So you can build your scene accurately and have your models fit into your 2D final image. I have a few cameras, and I had to look up this info for all of them.
On the plus side, in blender you can add the camera info as defaults, if you're recreating a shot and/or working with a particular camera.
There is already a great list of cameras in blender by default, but I recommend adding yours.
If you don't know the sensor size and other info about your camera, wikipedia tends to have this information. If not wikipedia the documentation that came with your camera, or google.