Obsessions: Synopsis and Effects
Obsessions are defined as intrusive and recurring thoughts that an individual finds disturbing or uncontrollable, and are not a good thing. 

There is a lay use of the word which means something like "a very strong interest," but it is just that: a perfectly normal if very intense interest, usually with no basis in abnormal psychology.

Note that 'fixation' is an outdated Freudian concept, unless, of course, you're talking about physically fixating your eyes on an object or area of visual space. Obsessions can be in almost any form, but common ones include recurring and distressing curse words, or a desire to perform dangerous or inappropriate acts (such as a spontaneous desire to jump out of a moving car, or to touch a nearby person in an intimate place.) Importantly, these are distressing and uncontrollable--the obsessive person does not desire these thoughts.