The small insects are called Drain Flies, a Psychoda species. Other common names for them are Moth Flies and Filter Flies.

Drain Flies are most commonly found indoors near moist areas, such as sinks, wet-mops that have been wet for several days, or in damp basements. These flies are poor fliers and are often found near their source.

Potential sources outdoors include a nearby area that remains moist for an extended period. Places to investigate as a source include roof gutters clogged with debris, at the base of downspouts, a wet spot near a leaking spigot, moist compost, and/or dirty garbage cans.

Drain flies are considered to be nuisance pests because they don't cause damage to people, their pets or belongings. The remedy is to locate and eliminate their source. No pesticides needed. A wet-dry shop-vac could help decrease the number of the adults but will do little as long as the moist breeding site remains.