"Silver ring" consumer lenses like e.g. the EF 28-90/4-5.6 or the EF 28-200/3.5-5.6 represent the bottom end of the Canon lens line-up. They're very cheap, the mechanical construction is simple (usually featuring a plastic mount) and the optics... well, let's say they all produce pictures. The AF motor is either a standard micro motor (usually DC type) or a micro-USM. The lenses have a silver colored ring next to the focus ring and are usually sold in kits. The silver ring "design" is new so there're a couple of older bottom-end lenses in the current line-up without silver ring such as the EF 80-200/4.5-5.6 II.
"Standard" type lenses represent the classic consumer segment. With the exception of the EF 50/1.8 these lenses have a metal mount and better build though there's still much plastic involved. The optical quality of these lenses is usually decent - some of the fix-focals such as the EF 100/2.8 USM macro perform stellar and are optically comparable to L class lenses.
TS-E "Tilt-Shift" is not exactly a quality class - these lenses are a very special breed though. They are the only lenses in the EF line-up without AF - this is indicated by the "E" (as opposed to "EF" - Electronic Focus). "TS" means "Tilt-Shift" which is a feature that allows a selective modification of focus plane (shift) and perspective (tilt). These lenses are especially interesting for architecture. The TS-E 24/3.5 is actually also an L lens.
L "Luxury" lenses represent the profession segment - both regarding optical and mechanical construction as well as regarding the price. L class lenses feature either UD, SUD, CaF2 and/or ground aspherical glass elements (see below). L class lenses have a red ring next to the filter thread. Against urban legends L lenses aren't always white colored - this is limited to long tele lenses - probably for thermal protection because these lenses are often exposed to the sun for several hours e.g. during sport events.
There're now 2 DO "Diffractive Optics" lenses in the line-up. DO lenses aren't better (but slightly worse) than L grade lenses but their major feature is being SUBSTANTIALLY smaller and more light-weight. Unlike L class lenses they've a green ring next to the filter thread. Thanks to a newly developed optical element ("diffractive") the DO lens is about 30% smaller and lighter than comparable standard lenses in its range. Unfortunately the price didn't shrink by a similar factor but increased by about a factor of 2 compared to similar "classic" design. Optically and mechanically the lenses are on L class level. The DO technology is unique to the Canon EOS system.
EF-S "short back focus“ lenses are the latest class of lenses by Canon. EF-S is no measurement for quality - it's defines that these lenses are specifically designed for the very latest APS-C (1.6x crop) D-SLRs starting with the EOS 300D (Digital Rebel). EF-S lenses are NOT compatible with film SLRs nor older APS-C D-SLRs or D-SLRs with bigger sensors. The rear element is a little protruding so these lenses require a dedicated (retracting) mirror design. The image circle covers the APS-C format only. This design allows extreme wide-angle lenses with a relatively long zoom range and a comparatively decent performance.
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from :
http://www.photozone.de/8Reviews/canonFAQ.htm
