Canada welcomes visitors, and depending on which country you are coming from, you may or may not need an entry visa. You will, however, need to ensure that you have legal status in Canada, whether you are a student, temporary worker, long-term visitor, or if you are immigrating to Canada. Almost everyone is welcome to visit Canada, provided they have a valid passport, are in good health, plan to return to their home country at the end of their visit, have adequate funds to cover expenses while in Canada, and don’t present any security or criminal risk to Canadians.
It is possible you might also need a Temporary Resident Visa, a medical exam, and a letter from someone you know in Canada inviting you to visit. A list of countries whose citizens are required to first obtain a visa before visiting Canada is available on the Citizenship and Immigration Canada website: http://www.cic.gc.ca/english/visit/visas.asp. The website is also a trove of information for anyone relocating to Canada, whether temporarily, or permanently.
When traveling to Canada with children (minors under the age of eighteen), you must have a valid passport and birth certificate for each child, along with a copy of the custody agreement/papers if you are divorced or separated. Canada has very strict laws about the protection of children.