The aphid group includes all aphids, blackfly and greenfly and are definitely FOE.
They vary in colour from green, pink, and yellow, black, greyish-white to brown and are about 2mm long. Most of us would be familiar with greenfly on our roses or blackfly on our broadbeans.
Aphids are sap sucking insects and produce honeydew that sooty mould can form on. They attack exposed parts of plants and roots.
They can spread virus and ants farm them.
Companion planting can encourage beneficial insects and predators such as hoverfly and lacewing larvae into gardens.
Insecticidal soap is very effective but kills beneficial insects too.
Protective cloches can be places over crops to prevent aphids infesting them but as they're so tiny they would be very difficult to stop. Squashing them between your fingers or spraying them off of plants with a blast of water from the hose works well too but you have to check the plants daily.