In the United States Air Force an airman basic is the lowest enlisted rank with a pay grade of E1. They wear no insignia on their sleeves and are addressed as "airman" regardless of gender. It is abbreviated AB.
An airman basic is an apprentice who is acquiring basic military and technical skills including the customs, courtesies, and traditions of the Air Force.
An airman basic is an OR1 on the NATO ranking system -- comparable to an Aircraft(wo)man of the Royal Air Force.
United States Military Ranks → Airman
A cadet airman basic is typically a newbie in their first few months who can't find their ass with both hands and a flashlight -- or at least not to the satisfaction of their flight sergeant. Typically a very frightened twelve-year-old boy who (1) got yanked in by a friend after a recruitment ribbon, (2) thought CAP was the quick 'n easy way to being a fighter pilot, or (3) was forced into it by a parent. During their time as a basic, the cadet will learn the basic drill maneuvers and memory work as well as customs and courtesies.
A basic wears silver CAP insignia on both collars since they haven't earned their stripe yet.
A cadet airman basic is addressed as "cadet" and is abbreviated C/AB. They wear no ribbon on their dress blues (if they have them, which they usually don't).
To advance to airman, a basic must pass a closed-book test on the first chapter of the leadership handbook including stationary drill with a score of 70% or higher, complete a cadet physical fitness test (PFT or CPFT) satisfactorily, and attend a moral leadership class.
Civil Air Patrol Cadet Grades → Cadet Airman
Wikipedia: Airman Basic, Comparative military ranks
Grunt's Military: Air Force Enlisted Rank Insignia
Leadership: 2000 and Beyond