what causes smell?

- a random thought that popped into my head last night as I was falling asleep. I wondered if it is always a bacteria that causes the smell. From what I read, I gathered that odors are caused by either bacteria or chemicals. Bacteria: body odor, bad breath, the smell after it rains. Chemicals: flowers, bread (if I understand correctly that yeast is a chemical).

I found these articles:
'Why do feet stink'
How does the sense of smell work? What causes a smell?
Why do flowers smell?

I thought I was researching a simple question, but while delving for the answer I found out so much more. You can't discuss this topic without knowing some basic chemistry (most of which I have forgotten!).