In general, it is the process of making something canonical-- that is, in conformance with some specification(according to rules). To canonicalize is to ensure that data conforms to canonical rules, and is in an approved format. Canonicalization may sometimes mean generating canonical data from noncanonical data(not according to rules).
Things to be taken in consideration:
- Pick the main URL & use it consistently in the website.
- Use 301 redirect to main URL.
- Don't remove www Vs non-www URL'S
Other ways that urls get canonicalized:
- keeping or removing trailing slashes,
- trying to convert urls with upper case to lower case,
- removing session IDs from bulletin board or other software.