As others have said, many shows have separate categories for hand and machine-quilted (and pieced!) quilts; some break things down further into pieced vs. applique and other categories (by size, etc).
Judges look at many different factors: the design, fabric selection, accuracy of piecing, quilting, binding... and depending on the show will rate them on various scales.
If you are really curious about how the judges figure out what quilts to award ribbons to, volunteer as a "white gloves" assistant during a local guild show that's being judged. I am planning to do that at our next guild show. Your job is to display the quilts for the judges, and you get to hear them discuss each quilt -- that will give you the best insight into what judges look for when they examine a quilt.
The last juried show I entered I got solid 4 out of 5 in all the categories (a "B", LOL!). I'm hoping to do better next time :)