The term "fitrah" seems to have some connotations of "natural" and is sometimes rendered as "innate nature". It crops up in different contexts, sometimes as including a natural inclination to the doctrines of Islam (see first link), circumcision, matters of hygiene and grooming. Wikipedia has a very small article on it that talks about it in spiritual terms. The question whether certain acts are against fitrah has been asked on here (in particular, homosexuality), and islamQA for example says that's the case.
That is quite a number of concepts grouped together into fitrah, so that leaves me with quite a diffuse understanding of the term. So my question is, what exactly is fitrah?