Karthikeyan isn't wrong, but it also depends where in the world you are. Loo is used in the United Kingdom, Australia and New Zealand, but not the Americas... although many people will still understand that you are talking about the toilet. 

Washroom, Bathroom, and Restroom are all used in Canada and the United States to mean the toilet. In the America's, unlike the UK, toilets, and baths or showers are commonly in the same room. In the UK, if you ask where the bathroom is, they will think you are talking about the room that has a bath or shower and is often a separate room from the toilet. No matter where you go in the western world these days, most toilets will also have a wash basin/sink in the same room. 

In my opinion, in Britain, as a young adult:

loo - toilet, most commonly used (UK only, not USA) The name of the room in general

washroom - very old fashioned, as a young person I would never use it. Ever.

bathroom technically contains a bath

In my family we call the room with a loo and a shower the showerroom but many others would call it a bathroom regardless.

Also to add on to Karthikeyan's point, 

A washroom is often the term used in a public setting whereas bathroom is used in private and home settings (as in if you are in someones house) 

the "loo" is a very casual term that refers to a toilet, this phrase is only common and understood in the UK, more specifically in England. it hold the same meaning as a washroom or restroom as Americans like to call it. 

Though in conversation, people may use any or all of the phrases depending on personal comfort levels, for example even in a restaurant you might hear someone say where is the bathroom (despite there being no bath or shower) but it is still accepted and understood casually. 

other phrases you might hear people use are

- Lavatory (usually higher end/class people will use this phrase) 

- water closet 

- little boys/girls room

- powder room (usually used in high class events as ladies would use the large lavatories to adjust makeup) 

- "john" You will also only hear this in the UK as its a slang word for toilet, credited to john harrington (Jon snow's descendant if you watch game of thrones haha) for inventing the flushing toilet. 

Sean:  "john" You will also only hear this in the UK as its a slang word for toilet, credited to john harrington (Jon snow's descendant if you watch game of thrones haha) for inventing the flushing toilet. 

John is a common term over here, Has been for decades.

Head is not specifically British either. It's also used in the US Navy and as far as I know, on all boats.

Crapper is another term for toilet. There is a myth that a man named Crapper invented the flush toilet.

'The facilities' is another term. Not heard often.