Colloquially, IMG means US citizen who has graduated from an MD program not based in the United States. For the most part it means people who did medical school in the Caribbean, although some will come from Poland, Hungry, Israel, Australia, etc. FMG refers to non-US citizens who have completed medical school outside of the US. These are physicians from literally every corner of the world who have trained elsewhere but emigrated for a variety of reasons. While the definitions blur a little and certainly overlap and/or are confusing, there is a distinction between the different training pathways.
Everyone has their own biases, and personally, from a purely academic/student/medical knowledge standpoint, there is a substantial gap between FMGs and IMGs. In general, it is as hard or harder to get into medical school overseas and while nepotism/money plays a bigger role overseas for some, there is little question that you have to be a very strong student to get into medical school.
As for my experiences. I am in a large, very diverse city. The physicians on staff at our hospitals come from just about everywhere. There are very few that took the IMG route. It is far more common to hear about someone doing their medical school in England, Egypt or India and their residency in the US than the Caribbean.