I've heard the Mandarin language called both Putonghua and Hanyu. What is the difference between these two words? Are they interchangeable ? Thanks!

Hanyu means Chinese language, meaning it can include other dialects i.e. Cantonese Putonghua is mandarin and is what's taught here. So really it's more accurate that you ask "Ni hui shuo putonghua ma." instead of "ni hui shuo hanyu ma?."

I am not Chinese but from what I've seen I'd say it is more a cultural ethnic reason for all the different terms. putonghua= 普通话= common speech aka the most accurate term for what we study here, but maybe not the most widespread term amongst Chinese speakers. hanyu= 汉语= language of the Han. The Han are the main ethnic group in China composing 90%+ of the people. This word I also believe is more referring to Mandarin, but it can also refer to Chinese language as a whole. Finally, you might get a funny glance if you use this vocab in Taiwan. guoyu=国语= putonghua's common name as spoken in Taiwan. zhongwen=中文= the general catch all word. Due to the wen 文 character it technically should more refer to written language but common usage doesn't always follow the rules! I have noticed that my friends who come from areas that have their own dialect much different than Mandarin (shanghai and changsha city in Hunan to name two off hand) will more oft call Chinese as 中文, while the Beijinger in the group likes to call it hanyu. So again putonghua and guoyu are terms specific to PRC and Taiwan, respectively. Zhongwen and Hanyu seem to be more subjective. Hope to hear from Lin on this one!

I had a chance to ask this question to a Chinese guy last night. He told me, like you said, that both words mean the same thing;however,he seemed to think that Putonghua was used more often than Hanyu. This may have just been where he was from. Maybe Hanyu is more commonly used in other parts of China.

putonghua = mandarin= common speech in China= official language in mainland China. hanyu=zhongwen, can be either Cantonese or Mandarin. Shanghai or Changsha dialects are called Shanghai hua, or Changsha hua respectively.They are hanyu or zhongwen, but not putonghua or mandarin. Guoyu is Mandarin, it is also putonghua, except it is a special term used by Taiwanese.

zhongwen

This is what I have learned: Hànyǔ (汉语) = spoken Chinese (usually Mandarin) Zhōngwén (中文) = written Chinese Pǔtōnghuà (普通话) = Mandarin yǔ (语) = language wén (文) = culture, writing The same can be applied to some other languages, i.e. Yīngwén is written English and Yīngyǔ is spoken English.

putonghua is the "official" language throughout China. It is taught in the schools, used on CCTV and other media throughout China in addition to other dialects that may be spoken in different parts of China. It means "common speech". As in other nations in which there are different ethnic groups, there is resistance to one language superseding regional identity and culture.

Tom U is correct Hanyu is Chinese language

Based on people I've talked to in mainland China, whether the people say Hànyǔ or Pǔtōnghuà when talking about Mandarin seems to vary based on where they live. Does anyone know which parts of mainland China prefer one or the other? 

I have many Chinese fiends most of whom speak Cantonese and some Mandarin. All of those from HK or southern China when speaking to each other (and me) always use the term Guoyu. However as mentioned by others, Hanyu or Putonghua are understood by most. Even those that do not speak Mandarin know how to respond to Ni hui shuo putonghua? (or Hanyu ma) 你会说普通话？还是汉语吗