If you own a digital camera and you need to change your lenses from time to time, the chances are, you will eventually acquire some degree of dust particles on and/or around the interior of your camera. For this reason it is important, that when you change the lens, you do so as quickly as possible, in a sheltered, out of the wind area, with your camera body facing down, so as not to allow any foreign bodies to enter. And you don't want to be spending half an hour looking for your replacement lens or body cap whilst your camera is exposed to the elements. For safety reasons, have it so that the task will be done within a couple of seconds at least. Even then, it is inevitable that some dust particles will manage to some how find their way in.
So, how do you know when you have dust particles inside your camera and in particular on your CCD/CMOS sensor?
You will probably first notice it after you have taken a photo that contains a lot of clear blue sky or a whitish wall, for instance. The dust particles will show up on your prints and/or on your computer screen as pale fuzzy little objects. You may see little black particles when you look through the view finder of your SLR, but they are not on your sensor. They will more than likely be on the mirror, which is situated just in front of the sensor. These particles, although they don't look too good, will not affect your photography.
"DO NOT", I repeat, "DO NOT" attempt to clean the inside of your camera on your own, unless you have had full instructions on how to do so. Without proper knowledge you can do a lot of damage with a minimum of effort. It is always advised to have the camera cleaned professionally, but if the situation is such that you cannot wait for that, if you have your camera's operator's manual, that should tell you how to do it safely, but follow those instructions implicitly.
The particles could be dust or they could be airborne pollen. If they are dust particles, a couple of blows with a photographic puffer (and "NOT" the pressurized aerosol type), could be enough to remove them. If they remain, then there is a strong possibility that it is pollen and you will have to call in the experts. Pollen will actually stick to the sensor and no amount of blowing will remove it successfully.
The CCD sensor is actually the bit that processes your digital pics, so the last thing you want on that is scratches or finger marks as they will definitely show up on all of your photos.
When you know you have debris on your sensor.
When you clean the front element of your lens you should also do it with great care. The lens element is coated with material that is also quite sensitive, but with care you can clean them very effectively.
It is advisable to go to a reputable camera dealer for all your camera accessories, as they can not only advise you on the best suitable products, but also explain properly about how to use them.