IMG stands for International Medical Graduate. In the world of medicine in Canada that typically means someone applying for residency who got their medical degree from another country.
In Canada, IMGs typically fall in two big categories:
Immigrants to Canada who were physicians in their country of origin and want to get licensed to practice in Canada
Canadians, who grew up in Canada and were unable to get in to medical school in Canada so get into an international medical school and are looking to come back to Canada for post-graduate training.
There are always people who fall outside these two groups (which I have made some generalizations in) but the above will encompass the majority of people who apply to CARMS as an IMG.
In my personal opinion the above two groups are phenotypically very different people. Group 2 clearly has an advantage on group 1. They are more fluent in English and have a strong appreciation of the Canadian culture and medical system. They have more contacts in the medical education system in Canada as most have done their undergraduate degrees from Canadian universities. Perhaps for the above reasons, they are also more confident during interviews.
A question I get asked often is whether having Canadian citizenship or permanent residence puts IMGs at any added advantage in the CARMS match. This is an important point to talk about as this is a major difference between the residency application between the US and Canada. To apply for a residency position in Canada you MUST have either citizenship or PR status in Canada. It is a requirement, not an advantage. For other eligibility criteria check out https://www.carms.ca/match/r-1-main-residency-match/eligibility-criteria/
CARMS typically separates IMGs and CMGs (Canadian Medical graduates) in the first round of matching. Which mean all IMGs (both group 1 and group 2 and any other IMG) are competing with each other for the few seats that are marked for IMGs only. After the first round of matching (more on this later), any unmatched seats in the IMG pool and any unmatched seats in the CMG pool get put in one group and unmatched candidates all via for them in the second round. Here its free for all and IMGs and CMGs compete with each other for these common seats.
So just by reading the above information you may have come to appreciate why Canada is a difficult land for IMGs. There are a very limited number of seats ear marked for IMGs in every speciality. There is a large number of IMGs competing for those seats. Contrast this to the USA where all seats are open to both local and IMGs so there are just much more opportunities to match as an IMG (mathematically speaking). There are some other core reasons, but more on that later.