Canon company tends to make lenses that fit into one of the following categories: cheap, better or best. This makes it fairly easy to read the letters and decipher the quality of the lens. A cheaper lens can still be good quality, but make sure it is appropriate for the camera and usage you have in mind.
Here are the basics to make sense of the codes unique to Canon brand.
First off, know your camera’s image sensor format and crop factor. See explanation
EF vs. EF-S format
Canon’s top-of-the line cameras have a full-frame sensor that doesn’t crop the image. In that way, they are similar to traditional film cameras.
EF mount lenses can be used on any DSLR cameras.
EF-S mount lenses only fit the cheaper APS-C format, cropped sensor cameras. So these lenses tend to be more cheaply made.
Ultrasonic motors (USM)
USM technology greatly improves the focusing quality over previous Canon lenses. It also greatly increases the price.
Image Stabilization (IS)
Check the age of your lens. The first IS systems didn’t do much besides burn up your battery power. Newer technology is much better. Test your own IS lens to see how it works with your photography style. IS technology may be added to a lens of any quality or type.
These are Canon’s top-of-the-line lenses, built with professional-grade durable materials. They are also marked with a famous a red ring around the lens.
If a lens has the II designation, it’s identical to the previous model but has improved weather-resistant construction and improved anti-reflective surfaces.
+ Lenses that open to f/2.8 and f/4.0 are more desirable than those with variable zoom apertures.
+ Lenses that collapse and extend into the body of the lens have more problems with dust and are more cheaply made.
Canon is constantly adding new technology and designations. There is currently no user-friendly site with a complete list of their codes. It works best to do an internet search for specific questions you have.