There are a few different causes of spots on the teeth, and I would definitely have your dentist
check them out if you have not done so already.
First, if the spots have been present since you were a child, this may be due to some disruption in the enamel formation at that time. Enamel hypoplasia in newly forming teeth is very common, as is hyperfluorosis, white spots caused from ingesting too much fluoride. If the spots are new and appeared on fully formed adult teeth, then these causes are unlikely.
Another potential cause is plaque formation. Plaque is debris and bacterial that form hard scale on the surface of the teeth, usually near the gum line. When present, plaque can make this portion of the teeth look discolored, and this may be what you are seeing.
Finally, the beginning stages of tooth decay or cavities can sometimes look like white spots, because as the bacteria produce acids that digest and thin out the tooth enamel, the enamel may begin to look white.
I would definitely suggest that you set up a visit with your dentist. They will be able to take a look at the spots and figure out if some treatment is needed.