One thing everyone agrees on, which is important to understand so I'll say it again later:
If you aren't 100% comfortable cleaning your sensor, take it to a camera shop!
The sensor is the most expensive, and the most delicate component of a camera. Therefore, trying to clean it with a cheap kit yourself may cause irreparable damage to your sensor.
These are just tiny specks of dust, on (/on top of) your sensor. To prevent them, be very careful when you switch lenses:
- Point your camera body downward when swapping lenses.
- Always keep a dust cap on the camera body (without a lens, even when swapping straight to another one) and a back cap on the lenses you're not using. This is a really good habit to get into.
- (Obviously) Try to avoid changing lenses anywhere with a lot of particulates (dust) in the air, or on surfaces.
- Don't swap lenses unnecessarily, and try not to do so "in the field" (see #3).
I used to get these little buggers all the time. Now I can no longer afford glass, so it's not a problem (no extra lenses = no swapping).
You can also clean your sensor, but be aware that the more you do this the shorter your sensor's life will be and it must be done properly with the utmost care! Pixels die surprisingly fast with excessive cleaning, and sometimes (if you are in a dusty environment) you will attract new dust even as you remove the old dust.
Professional cleanings are advisable unless you know what you're doing (there plenty of tuts available online though, and it's not rocket surgery). Another way to deal with this is to use a "dust map" with software that will let you use it. This was more applicable to older cameras, and some newer ones actually do this "behind the scenes".
To check for dust after a cleaning
You will need:
- a consistent white (or very light gray 50%-) surface
- a lens with a focal length of 50mm+
Then you will need to:
- Set the lens focal length at 50mm or longer. You want the white
surface to completely fill the frame.
- Switch the camera to manual
focusing (either a switch on the lens, on the camera, or in a menu).
- Set focus to infinity (turn focusing ring counterclockwise until it
reaches the farthest end of it's focal range).
- Hold the camera about 8-12 inches from the surface.
- Take a picture.
This should show you any dust/debris on your sensor (invert the image to increase visibility). It is also the method used to create a "dust delete" image if your camera supports them. Also, of course, you can remove these in post-processing with Photoshop.
Here's an article on sensor dust, and how to deal with it from Digital Photography School.
...and here's one from "dummies.com" (specifically about a Canon 60D, but most of the advice is universally applicable).