USAF is short for the United States Air Force — the branch of the American armed forces that flies aircraft, and now also handles missiles and much of the military's satellite and space work. You say the four letters one by one, "you-ess-ay-eff", never as a single word. It appears mostly in writing: news reports, military documents, the side of an aircraft, a job title like "USAF Colonel James Reed". In speech people are much more likely to say "the Air Force" and let the country be understood from context. Note the "the": you say "he joined the USAF", not "he joined USAF", because the full name has "the" in it. Careful — plenty of countries have an air force, so USAF specifically marks the American one, the way ROCAF marks Taiwan's.