Code Smells

I believe that writing a well structured code is one of the hardest important capabilities that a programmer should develop to achieve high quality code, however, sometimes we are forced to code dirtily since time equals money, not to mention how hard is it to understand and modify someone's code who didn't think that others would read it.
Code Smells are some rule thumbs that helps pin-pointing to a location of weak code, some code smells are contradictory, but learning them will help pointing to a code which needs improvements, and more important, keeping them in mind will prevent you from writing stinky code in the first place.

Here you can find a list of the most frequent code smells, simply and well explained, and here you may find some more detailed info.