What are "tensides"? I was reading about chemicals that control pore size of polyurethane foams and there it was mentioned that "tensides" decrease pore size. What are tensides? I assume it's the opposite of a surfactant, that is, it increases surface tension, but that is just my guess. I have no idea what it actually is.

I tried to Google it but strangely I just got bunch of "surfactant" results even through I wrote it just tensides. Some German site when translated from German to English changed the word tenside to surfactant, so I don't know if it is different class of chemicals or just a German word for surfactant. From the original article where I saw the word, it was written in a way as to suggest that they both are different from each other.
I don't remember to have seen the term tensid outside the context of the German language. 

The term is typically applied to agents that decrease the surface tension (hence tensid) of water. Note that the logic behind the English equivalent surfactant is just the same.

So, in summary tensid and surfactant mean the same.