A chum asked for some clarification about the glib remarks on Queen Elizabeth II's role in effectively vetoing legislation. It came up in the posting on the ex-televangelist's drive to get her to do that with Canada's same-sex-marriage bill C-38.
It's a little out of our ken down here. You can get enough detailed background from Wikipedia's article on Royal Assent.
The gist of it is that for Commonwealth countries, including Australia and Canada, the ruling monarch is the titular head of government. In theory, the Queen could keep a law from taking effect by withholding her approval. This is part of the monarch's Reserve Powers, which are very rarely exercised.
Notably in 1937, two bills giving Alberta provincial, non-federal banking authority, and one bill requiring newspapers to print government rebuttals fell afoul and did not receive assent. It's possible too that if Quebec tries to separate, such a bill would not become law.
There is a bifurcation here though. In the U.K. (England and Northern Ireland), legislation comes directly to her. In places like Canada, the Governor-General is her representative and almost always takes the advice of the ministers. In practice, unless she tells him otherwise, he assents to the bill and it is the law of the land.
There are also some delightful rituals, passing of papers, separate notification of the houses of Parliament and so forth. It's a lot better theater than our Congressional snuff boxes.