It's very common and not the same as in Spanish.

In class I asked you to notice where it is placed (e.g. both ends right before and after the words or both ends or both ends with a blank space before and after the words)...

And I told you about its use as signalling that you are going to expand on an idea -- by linking it to another idea or its opposite/contrast, by expressing an afterthought, by offering more detail so that it is clearer or more informative... (It is similar to some uses of the Spanish color and the Spanish brackets.)

Check this out (Collins Punctuation Glossary):
http://www.collins.co.uk/wordexchange/Default.aspx?pg=71#