If you own a Digital SLR at some point you will look at your photos and wonder why there are little black spots in the same position on some of them. Often people think that it's a mark either on or inside their lenses - which it's possible it could be but if you have multiple lenses it's very easy to rule this out, whilst I've even heard of some people thinking that the sensor on their camera has been damaged and have sent it off for repair.
In 99% of these cases the cause of these black marks is simply dust on the sensor. Even if you don't currently have any marks on your photos there is still a very good chance that you have dust on your sensor too. To see if you have try this simple test.