Because a DSLR allows the user to change lenses at will, dust can eventually enter the mirror chamber of the camera and deposit either on the mirror or the sensor. Sometimes the dust is visible on pictures.
This was not a problem with film because each film section was used only once, so any dust particle depositing on one section of the film would not affect other sections.
A gowning number DSLRs offer multiple dust removal features. These include anti-static coatings to repel dust and vibrating sensors to remove it. However, sometimes a few particles of dust will remain on the sensor. When this happens, the dust must be removed by the user.
The best way to remove remaining dust is with a rocket blower.
Rocket blowers target specific areas on a sensor
Rocket blowers direct a thin flow of air on the targeted area of the sensor and allow dust to be removed without touching the sensor itself. Rocket blowers can be large or small, but one feature that’s important to have is a valve preventing dust to enter the blower chamber. Known as an air inlet valve, it is designed to prevent air back flow from the air nozzle.
Most of the time, a rocket blower will take care of all dust particles deposited on the sensor. In those cases where particles can not be removed, have the sensor cleaned professionally if you don’t have the skills to clean it yourself.
Rocket blasters by Giotto are among the most popular rocket blowers.