Adeno-associated virus (AAV) vectors are largely gutted-out versions of a type of single-stranded DNA virus that typically remains episomal (doesn’t integrate with the genome) and when it does integrate, it tends to be at okay places. A great feature is its low immunogenicity, but a large disadvantage is its small size. It can only deliver up to ~4.7 kb of DNA, but strategies to overcome this include use of helper viruses that allow additional genes to be removed from the viral vectors and the corresponding proteins provided by other viruses cultured alongside them during lab production. You might see the term “rAAV” which stands for recombinant AAV, which is a version of AAV genetically engineered to be optimized for therapeutic purposes.

P.S.: These viruses of the parvovirus family have “nothing” to do with adenoviruses other than they were first discovered as a contaminant of adenoviral culture!