As the name implies, they are white and fly around. When disturbed they are most easily noticed flying from plant to plant.
Their larval stages are very similar to aphids in that the suck plant sap, form little groups and secrete honeydew. This honeydew similarly attracts ants and sooty mold. They differ from aphids in the scale like pupation stage that may be visible on the underside of leaves as seen above. Soft plants are most susceptible.
Large infestations will lead to yellowing leaves, black messy blotching (due to sooty mold) and eventually falling leaves.
Whitefly are hard to eradicate once established and may end up requiring an insecticide spraying program repeated every few days and lasting several weeks.
Some success can be had using a handheld vacuum to remove the critters.