Canon’s L-series is Canon’s premium/pro lineup. In general, L lenses are designed with better materials, more advanced optical formulas, and stronger construction than non-L lenses. Common differences include better build quality, more frequent weather sealing, higher-end autofocus systems (often ring-type USM with full-time manual focus), and features like fluorite/aspherical elements to improve optical performance.

That said, “L” is not a guarantee that every L lens will outperform every non-L lens in every respect. Some non-L lenses can deliver image quality close to, or occasionally better than, certain L lenses, depending on the specific models being compared.

In practical use, the main advantages of L lenses are often durability, handling, autofocus performance, and consistency, rather than image quality alone. They’re also typically aimed at full-frame EF users; historically there have been no EF-S L lenses.

A small but real difference: L lenses typically include accessories like a hood and case, while many non-L lenses do not.

So the answer is: L lenses are usually better overall, especially for demanding or professional use, but the amount better varies a lot by lens.