Most likely you recently brought house plants inside for the winter. If you also seem to have a sudden rash of fruit flies, well, maybe not. They may be fungus gnats.
Fungus gnats are common in house plants and usually not much of a problem. They are tiny, about the size of fruit flies, so many people think they have fruit flies when they actually have fungus gnats. The gnats can be found flying near or on house plants and lay their eggs in the soil. Adult gnats do not harm plants, but, in a large infestation, larvae can cause significant damage to plant roots. This is more of a problem for commercial greenhouses than for the home gardener.
Fungus gnat larvae need wet soil to survive. The best way to control fungus gnats in houseplants is to allow the top couple inches of soil to dry out between waterings. Keep water drained from saucers under pots. The problem should resolve in a few weeks.
http://www.ces.ncsu.edu/depts/ent/notes/Urban/fungusgnat.htm
Fruit flies eat decaying produce and are most often seen in the kitchen or other food storage areas. To control fruit flies, throw out any produce which has rotten spots. Cover kitchen scraps waiting to be composted, rinse food containers before recycling, and empty the kitchen trash can regularly. Don’t leave dirty dishes sitting, and keep the kitchen clean, especially the sink and drain. Even damp, unclean sponges and towels can support fruit flies, so change them often.
