Married Vs. Common-Law
Canada recognises to Common-Law partners the same rights as to married couples. When applying to most of the immigration programs, the main applicant is able to sponsor its companion and kids, regardless of whether they are married or in a common-law relationship.
Nonetheless, there are some differences that need to be carefully addressed.
Marriage
Marriage is a de jure relationship, meaning that it has been established in law. In order to be considered a spouse, in Canada, you need to be at least 18 years-old, legally married, and in no other spousal relationship with anyone else.
Also, as long as a marriage is legal in the country it was celebrated, and is conform to Canadian Law, then it is to be considered valid for immigration purposes (IRPR, s.2).
Common-Law
"common-law partner means, in relation to a person, an individual who is cohabiting with the person in a conjugal relationship, having so cohabited for a period of at least one year. (conjoint de fait)". A common-law relationship is legally a de facto relationship, meaning that it must be established in each individual case, on the facts.
Please, refer to 5.35 of the OP 2 Operation Manual used by immigration officers to assess evidence that a cohabitation exists and that a couple is in a common-law relationship.