Plasmids are relatively small, typically circular/ringlike self-contained loops of DNA that exist in the cytoplasm of prokaryotes (typically) as well as eukaryotes (less prominently). They can replicate and move independently from the chromosomal DNA.
One of the most significant implications for the existence and function of plasmids is that they make both evolutionary adaptation and genetic engineering easier. By making genetic information available outside of the constricting location and structure of a nucleus/nucleoid or the highly regulated and interactive chromosomal DNA, the organism not only can access new genetic material within its lifetime, but we are able to do so artificially without the potential complications of working with chromosomal DNA and disrupting those controls.
Transfer of plasmids between living organisms is called horizontal transfer, and is the primary reason for bacterial resistance in the case of medications; the more resistant bacteria attribute this property to plasmid genes, which are shared with other bacteria due to the resistant bacteria being over-represented in post-treatment survivor populations.