Chromosomal DNA
A prokaryotic cell has a single main chromosomal DNA molecule. This DNA is comparatively short, forms a circular molecule and is not associated with proteins.
The chromosomal DNA occupies the nucleoid area of the cell.
Plasmids
Prokaryotic cells also usually contain one or more plasmids. A plasmid is a much smaller circular DNA molecule carrying only a few genes.
Plasmid DNA is readily accessible to proteins involved in gene expression, so plasmids can carry genes whose products may be required quickly. Antibiotic-resistance genes are a common example.