As a rookie photographer who just learn about photography those codes written on the lens of your camera can be very confusing. There are some explanations on those written codes on canon lens here.
EF here stands for “Electro focus”, while the S stands for “Small Image Circle”. It is called so because this lens has its own motor electronic to find focus automatically while the small image circle is there because the image which is transferred to the sensor is smaller than the image size on the EF lens. EF-S lens is specially designed for a non full-frame canon camera (the sensor size is smaller) and cannot be used for full frame camera. Apart from that, EF-S lens is usually used for low-end class lenses.
EF – Electro-focus
This one is similar as the EF-S, yet this EF lens is not only designed for canon full frame camera (EOS 5D Mark II/III, EOS 1D Mark), but also for non full frame cameras. The EF lens has bigger image circle compared to the EF-S.
L – Luxury
High class lens launched by canon L series is designed using superfine quality optic material with more expensive price. This lens is intended for professional photographers or serious enthusiasts’ class. The L series lens is mechanically great and weather resistant. All L series lenses are in the same category as the EF lenses and can be used for full frame and non full frame cameras. This lens is marked with a tale-tell red band around the lens.
IS – Image Stabilization
Lens with IS which is completed with a technology that can afford to minimize blur by compensating hand movements when you hold the lens is very beneficial for low light shooting or when you use Tele lens.
DO – Diffractive Optic
Lenses with DO feature is designed to minimize chromatic aberration and reduce the lens’ weigh and size altogether. With the same focal length, camera lens with this feature can be two or three times shorter in size compared to lens with standard optic. This can be very useful for Tele lens so that the size is not too large.
USM – Ultra Sonic Motor
All canon lenses have motors inside them but if they are featured with USM technology, it means the lens has faster autofocus ability. Moreover, the sound is not so noisy and it can also save more battery and we can change from the autofocus to manual focus by turning lens’ ring only. Lens featured with USM is usually marked with a gold colored band around the lens.