What is the difference between 國語 and 普通話? I am still a beginner, but after 1 year I'm still not sure about the difference. I've been talking with some citizens from China mainland on the Internet, when I mentioned that I'm learning 國語, I seemed to offend people, and it sparked some rows in comment sections before.

I have lots of confusion. Now I know 國語 means "the national language", and 普通话 is "the common language", and is a standard. I'm learning based on Taiwanese materials, and they refer to 國語 as the language there, which is for all intents and purposes the same as 普通話. 

I'm unsure why it causes so much fuss. Is 國語 an unpopular term outside of Taiwan? If so it makes sense where confusion comes from.

My teachers and materials seem to make no real distinction, and both terms are even interchangeable from how I've been taught. I would love to have this cleared up.
「國語」is not an unpopular term outside of Taiwan.

It literally means national speech, and is used in places where「普通話」may make less sense. Since「普通話」literally means common speech, regions with populations which prominently speak other languages may avoid the term「普通話」to various degrees, because in those regions the common speech is not necessarily Mandarin.

You may hear「國語」in Hong Kong, Macau, and Xinjiang to refer to Mandarin, interchangeably with「普通話」. Although「普通話」is overwhelmingly more common in the Mainland,「國語」shouldn't incite a reaction there; if it does this could be due to various reasons, e.g.:


Some people may take it to be a form of Han/Northerner/Mandarin Chauvinism, aggressively discriminating against other topolects or languages as not being part of the nation;
Some people may have political interpretations of the term, and identify「國語」inappropriately with anti-CCP leanings (note that CCP decreed that Mandarin was to be called「普通話」some time in the 1950s, officially making the word「國語」obsolete in Mainland China).
You're on the internet, so be prepared to attract angry comments about everything and anything.


Another word for Mandarin is「華語」, which is used by Malaysian Chinese. This is appropriate in their situation because Mandarin is neither the national language nor the common language of Malaysia.

For something that shouldn't incite inappropriate reactions, consider using「漢語」or「中文」instead.