This article is from the FAQ, by with numerous contributions by others.
Permanent residents of the United States (Green Card holders) are,
like U.S. Citizens, exempt from the requirement to have a passport
or a Canadian visitor visa in order to visit Canada. The Green
Card itself is sufficient documentation.
The decision on whether to admit a visitor to Canada is made by an
Immigration Officer at a Port of Entry to Canada. This is the case for
all visitors including those who hold Canadian visitor visas. The
Immigration Officer also determines the length of stay. If a Green Card
holder is admitted to Canada without any document being issued to him or
her it means that the GC holder has been admitted to Canada for 6 months.
A shorter stay than 6 months would be indicated if the GC holder was
issued a document called a Visitor Record which specifies the length of
time for which the holder has been admitted to Canada.
Persons seeking admission to Canada at the land border do not normally
fill out any forms to apply for admission. Those arriving by air
complete only a Customs form. There is no fee for the admission of
a GC holder to Canada as a visitor.
GC holders, like all other persons seeking admission as visitors, can be
refused admission for a variety of reasons. The most common one is the
existence of one or more criminal convictions in the applicant's past.
The most common criminal conviction is Driving While Intoxicated or
Driving While Impaired. These offenses fall with the criminal law in
Canada and even though they are usually regarded a misdemeanors in the
U.S. they are regarded as criminal offenses in Canada.
Visitors to Canada may not study or work. As in the U.S., different types
of visas or Authorizations are required to study or work.