Bills are issued at 4, 8, 13, 17, 26 and 52 week maturities, plus cash management bills at odd tenors. They pay no coupon. You buy below par-value and receive 100 at maturity, so all of the return is price accretion.
Bills are the closest thing the market has to a risk-free cash instrument, which makes them the collateral of choice in repo and the benchmark for money-market funds. Their quoted discount rate is not a true yield, so always convert to a bond-equivalent-yield before comparing with a note.
Example: a 26-week bill is bought at 97.45 per 100. Over 182 days you earn 2.55 per 100, so the holding-period return is 2.55 / 97.45 = 2.617%. Annualised on a 365-day bond-equivalent basis that is 2.617% x 365 / 182 = 5.25%.
Related: treasury-note, treasury-bond, zero-coupon-bond, bond-equivalent-yield, treasury-auction