What is the difference between a bacterial artificial chromosome (BAC) and a plasmid?
BACs are a special kind of plasmid, based on E. coli's F-plasmid:

The BAC is based on a plasmid in Escherichia coli that is termed the F (for fertility) plasmid. The F plasmid (or F factor) contains information that makes possible the process called conjugation . In conjugation, two Escherichia coli bacteria can physically connect and an exchange of DNA can occur.
A BAC contains the conjugation promoting genetic information as well as stretch of DNA that is destined for incorporation into the bacterium.

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It isn't necessarily larger than a plasmid.