Are you aware of what the purpose is of the u-shaped pipe underneath your drains? This is called a p-trap, and it is a pipe that is designed to always have water standing in it. The water is there to stop sewer gasses from leaking from the plumbing system into your home. This is important because sewer gas can be very harmful to you, apart from smelling really bad. It also serves a secondary purpose by stopping small items from going down into your plumbing system. This has saved many a diamond rings when they accidentally fall down the sink.
Some p-traps come equipped with a plug that you can remove to access it and clean it out, or retrieve items that have fallen in. You should keep your p-trap clean, since it does tend to collect soap, hair and other debris.
P-traps are very simple in function, but at the same time, vital to your plumbing system.