Integrons 

Integrons are genetic elements that allow bacteria to adapt and evolve rapidly through the stockpiling and expression of new genes.
These genes are embedded in a specific genetic structure called gene cassette that generally carries one promoterless open reading frame + recombination site.
Or
Integrons are genetic elements that contain the site specific recombination system able to integrate, express and exchange specific DNA elements called gene cassettes.
Integrons are not considered as a mobile elements because they lack function for the mobility.
Integrons are consists of:
A regulatory region
An open reading frame encoding a site-specific recombinase
A target site for the recombinase