We have all noticed an odor before that might have puzzled us... Where is that smell coming from? Why can i still smell this? It's a real annoyance.
Every item that you smell gives off molecules, from the smell of meat cooking, to beautiful perfumes, to smelly socks. The odor molecules of the substance travel into your nose and your brain reacts and recognises the smell.
Volatilised chemical compounds that are produced by bacteria cause these odors. Bacteria produce horrible smelling gaseous molecules that our noses discover.

Common odor sources:
Food scraps
Bin Smelly paint
Pet fur
Moisture problems (mold, mildew, rot, etc.)
Dirty carpets Malfunctioning electrical wires
Pest infestations
Pet accidents
Smelly paint
It's very easy to eliminate an odor once you find the source. You can try to remove it or contain it. If a smell is hard to remove even when the item causing it is taken away, then open windows and doors to welcome fresh air inside.