Both experience and things I've learned through the course of my job. My pet articles especially are from experience, as well as things I've learned from a wide range of animal experts. For my job -- I am a writer, but often take transcription and note-taking work as well, and glean some very in-depth knowledge on some pretty amazing things that can later be incorporated into articles. I also write a lot of what I learn from acquaintances and my husband -- namely, any medical information that I don't get from the medical research note-taking that I do comes from my husband and his co-workers.
School...no, I really don't use anything I learned in school :P.