Is there any difference regarding the usage of detrimental and harmful? I am not a native English speaker, so I don't understand the different nuance of these words.
The two words have very close meaning. I would say that “detrimental” sounds slightly more conditional than “harmful”, but it might only be because it is a less common word.
It seems to me that harmful can cover a far wider range of harm, whereas detrimental has connotations of a minor effect that decreases the efficiency of whatever it's been applied to without stopping it from working entirely.
Harmful means (basing on what the NOAD reports) "causing or likely to cause harm"; detrimental means "tending to cause harm".
Harmful should be used for something that effectively (or probably) causes harm, while detrimental should be used for something that frequently (or regularly) causes harm.