Many people who have criminal records can not travel to Canada without special permission. The Canadian government has an overview of the rules, but many crimes which may seem relatively minor to you are included on the list.
Anecdotally, it seems that the Canadian and US governments share a lot of data here, so it can be difficult to avoid the rules. I know Americans who have traveled fairly extensively without incident after criminal mishaps in their youth, but who have not been able to go to Canada.
If you are in fact ineligible to enter Canada, there is a process to overcome this ineligibility but it is quite complicated and you may wish to hire a lawyer or someone else experienced with the process in order to help you.