Here, in Canada, "washroom" is the preferred term for a public room with toilets. Some people also use the term "restroom". However, you would never use these words to refer to the same room in your home, which, in that case, becomes a bathroom.
I use the word "loo" too, but most other Canadians often find it a wee bit funny, just like when I say "flat" or "naff". I didn't grow up speaking Canadian English, you see.
They get used interchangeably, but they all have different meanings.
A latrine generally refers to an outdoor toilet without running water, or just a pot that people would urinate in. Not used commonly in North America
A toilet is the actual object in which you pee/crap in.
A washroom is usually a room with a toilet and sink, but no shower or bath
A bathroom is a room with toilet, shower/bath and sink.
It's generally acceptable to use toilet, washroom and bathroom interchangeably
FYI:
When I said latrine was not used commonly in NA I meant the actual word.
There are plenty of outdoor toilets in rural areas and campsites, but they are usually called "outhouses"
in the usa, i've only heard "latrine" used in a camping situation. a wooden box with a toilet seat over a hole in the ground. it could have partial walls around it too, but it is not an encolsed room with a door like an "outhouse". if you asked for the "latrine" in any urban area, you would get a puzzled look.
sorry if that was already answered.
i agree with the explanation of Paul. Really u explained very well.