There are a number of reasons that there are white spots on your teeth. It can range from an issue called fluorosis, which means that a person has had too much fluoride and it has affected the tooth structure. What people forget is that many mouth washes have fluoride, major metropolitan areas have fluoride in the water,toothpastes, bottled water and on top of that some dentists do fluoride treatments after a cleaning. These things make it easy to get white spots on the teeth when you are a kid and the teeth are growing. There are other types of white spots- after wearing braces with wires, around the brackets, which were glued to the teeth people will get white spots. These spots are due to poor oral hygiene around the brackets. Poor oral hygiene will also cause white lines at the gum line of the teeth.
These are the common causes of white spots that can be reversed easily and with minimal invasiveness. What it requires is scrubbing the teeth, cleaning them with a gel and then using a calcium paste to force calcium back into the tooth, repeat if needed. It's Simple and effective. If this method doesn't work then you can move onto more invasive techniques like composite(bonding), veneers(porcelain facings) or decide that the spots aren't so bad after all.
There is one last type of white spot that hasn't been covered. Sometimes when a person gets a cavity the tooth can turn a dull white. This gives the patient and dentist no option but to treat the decay.