The difference is simple, Google Canada is localized search, while plain Google is international broad search.
It is better to focus on Google Canada for localized results. i.e. if you are selling real items like bicycles from Ontario you don't need your site to rank number one in Google, as a Japanese guy might be browsing but he cannot buy your product. While if you rank at number one for Google Canada or even more targeted search like Ontario web directories your business will bloom as people from Canada will see your product easily.
It is far easier to rank high for localized results, which is the first part of SEO if the product and services offered by a website, blog, hub are localized and not informational in nature.