1) The difference between "Canadian" and "U.S." English is much less than the differences in English between different regions within the United States. I'm a U.S. native. If I don't know that someone is from Canada, I can talk to them for half an hour before I notice anything "Canadian." Canadian Peter Jennings was the sole anchor on the big U.S. network news program "ABC World News" and many people were unaware that he was Canadian. 2) In friendly conversation or in a business or office situation, U.S., British, and Canadian speakers communicate effortlessly, and can negotiate details without misunderstanding. 3) I sometimes have trouble understanding rapidly spoken British English that's intended for British ears; it depends on social class and regional accent. But the same is true for U.S. regional dialects. 4) Wikipedia is the best source I know for details of U.S./British differences. 5) There are well-known minor differences but they are rarely important. In many cases, the supposed "British" usage is a familiar part of U.S. passive vocabulary, and vice versa. The difference between "colour" and "color" is obvious, but I was never even aware of "traveller" vs. "traveler" until it was pointed out to me. I say "railroad," but I understand both "railroad" and "railway" so easily that I don't even notice which one was said.

There are no 'countable' differences. You will find varying sounds: intonation, stress and articulation; vernacular vocabulary, and culture, which in itself is directly tied into the vernacular. However it's important to note that for the most part they do not differ hugely in terms of being able to be understood from one to another.

As far as accent goes, there is a continuum, and it depends on where people are from. There are many English accents, and it doesn't really matter which you learn, as you will be understood by all English speakers. Canadian English sounds more like American English (with the exception of the provinces of Nova Scotia and Newfoundland, which sound more Irish, and Québec, which can sound more French if people aren't native English speakers). As a Canadian, I have a hard time with some southern American accents; Louisiana and Alabama are two examples. Canada as the USA form a dialect continuum, meaning, as GregS said, that a Canadian and American living close to the border will sound more like each other than their compatriot from the other side of the country. For example, people from Vancouver (Canada) and from Washington (USA), which are very close to each other, sound almost exactly the same, where as they both sound nothing like people from St John's (Canada) or Baton Rouge (USA). As a linguist, I can hear some differences between Canadian and American accents usually, but to the untrained ears, those differences are minimal. In fact, the differences are so minimal that Canadian English is often classified as a type of American English.

Dan Smith has a great answer. Adding to that for US/Canada -- I feel it is more geographically related instead of by the border. I live in the northern part of the US, very close to Canada, and we all sound the same on either side of border. In the NE US, there is strong relation to the SE region of Canada. Etc. We do tease our Canadian friends that hey tend to add 'eh' at the end of sentences, "It's cold outside today, eh." (It's not a question.) But then, we have our own idiosyncrasies too, "Well it is winter, don't-chya-know." :-)

As you have already been told, the differences are not that extreme. We can all understand each other, whether British, Canadian, American, Irish or Australian, without too much difficulty. I am Canadian, but have lived in the UK (which, by the way, has different vernaculars within its borders). Between the three variants, there are some pronunciation differences (stress, intonation, vowel quality, rhoticity), some vocabulary differences and some variances in spelling. In writing, Canadian English is much closer to British English than it is to American. That is to say that Americans decided to change English spelling and they now have a number of unique spelling customs. They don't impede comprehension any, but if you see someone having spelled "colour" as "color", randomise as "randomize", or "centre" as "center", than that gives you clues as to their nationality (American variant being the second variant).