This document discusses high and low copy number plasmids. It defines plasmids as small, circular DNA molecules that can replicate independently of bacterial chromosomal DNA. Plasmids are classified based on their copy number per cell as either low (1-4 copies), moderate, or high (>100 copies). Examples are given of common plasmids and their typical copy numbers. The advantages of plasmids include being easily isolated, replicated, and having multiple cloning sites, while disadvantages include smaller size limits and replication rate decreasing with larger insert size.