Trademarks are indications of origin. A word in common usage can still indicate origin, it can still be distinctive enough.
For example consider the word "wooden". If I'm selling furniture it's not distinctive. If that's my trademark for computer software then it's distinctive.
Tradmarks are not the same as Registered Trademarks either. Anything you use to indicate origin can be a trademark.
A classification system ("Nice", http://www.wipo.int/classifications/nivilo/nice/index.htm?la...) is used internationally to specify the locus of registered trademarks. Thus I can register the Trademark "wooden" against software (class 9, 42) and you can register the trademark "wooden" for your legal advice centre (class 45). Neither registration stops me using the words unless in a context relevant to the mark where the origin of goods/services might be put in doubt.
[Note this is a primarily European view and doesn't account for all differences in TM law in Europe.]
Further reading:
http://www.uspto.gov/faq/trademarks.jsp
http://oami.europa.eu/ows/rw/pages/ctm/index.en.do
http://www.wipo.int/trademarks/en/
