Biology:

What is a plasmid? How does if differ from bacterial DNA?

Plasmids are small rubber-band-like loops of DNA that are separate from bacterial chromosomes.

One simple definition is that the chromosome is simply required for viability under all conditions. Plasmids are "required" only under conditions when the bacterium encounter a harsh environment. As such, you will always find housekeeping (required biosynthesis and catabolic) genes on the chromosome but may not find them on a plasmid.