Cavities result from demineralization in an acidic environment. Acidic foods and drinks (e.g. orange juice, lemon water) can directly cause demineralization over time. Bacteria in your mouth also metabolizes carbohydrates (especially sugar but also chips, crackers, etc.) into acid.
This demineralization is first detected as incipient caries. On visible surfaces, this shows as a chalky, white spot on teeth, especially around grooves and fissures in teeth. Between your teeth, this shows as a shadow in your enamel. At this stage there is still a chance for remineralization.
Eventually, as caries progresses past the enamel into the dentin, it reaches a point that cannot be remineralized (cavitated lesion or cavity). If you have bone loss from gum disease exposing the roots of your teeth, those teeth are particularly susceptible to cavities, because there is no enamel at the root. There is typically no pain at this stage. Some patients may experience sensitivity to sweets or flossing, but most have no symptoms at all. At this point you may need a filling or other restoration as prescribed by your dentist.
If left untreated, carious lesions get bigger much more rapidly once through the enamel. If allowed to reach the nerve, you may experience severe pain to hot or cold sensations. From this point on, you may need a root canal or extraction depending on the extent of decay.
If still left untreated, eventually the nerve of the tooth will die. Your pain may go away for a short while. Because bacteria now has a highway into your bone, you will develop an infection at the end of the root. Eventually the pain will return. You may experience a spontaneous, severe throbbing pain. You may also experience pain when chewing.
If still left untreated, the infection will get worse. In the best case scenario, the infection tunnels out the side; you may feel a small bump on your gums along with having a foul taste. Sometimes the infection may spread to adjacent teeth. Sometimes the infection may spread to a fascial space, causing swelling–this is a serious problem and may be life-threatening.