like...what the source video looks like before compression. What kind of compression scheme you use. At what bit-rate, q level etc etc.
There are all these factors that determine what your video will look like.
In a nutshell, MPEG compression will in most cases create "blocky" artifacts. To see these artifacts, look at the black areas on a DVD. You'll notice these "waves" so to speak of boxy mosquitos floating around. The source material won't have these.