Chinese is which one? 中文 (Zhong Wen) 華語 (Hua Yu) 漢語 (Han Yu) or 普通話 (Pu Tong Hua) ?

A foreign friend asked me what is the difference between 中文 (Zhong Wen), 華語 (Hua Yu), 漢語 (Han Yu), and 普通話 (Pu Tong Hua). Are they the same?  Generally speaking, they all mean "Chinese."

All of above sentences mean "I am learnng Chinese. (I am learning Chinese language)"

In TenTen's experience, "我在學中文" is the one most people use.
And the other are also very common and correct to Chinese ears
So no matter which one you use, Chinese people will understand you.

If you want to clearly distinguish them, try to undertand each character...

In ancient times, Chinese people thought their country is in the center of the world. Accordingly, they named their country "中國 (China / Central Country)" and their writing and language "中文 (Chinese / Central language)"

"Han" used to be a representative dynasty in Chinese history, so people use "Han Yu" to refer to Chinese language as well. And "Hua" also has its historical meaning in Chinese world. If you are interested in Chinese history, you can refer to: 

http://zh.wikipedia.org/wiki/华夏族 [Chinese Version] 

Different from 中文, 華語, and 華語 whicn can broadly mean all Chinese dialects, 普通話 is a more specific term. It refers to "Modern Mandarin Chinese"

In Taiwan, people also call 中文 as 國語 (guó yǔ). If there is someone who says "I'm learing Guo Yu," he means he is learning Chinese.