Food
Parasitic on plants, they drink plant fluids through their piercing, tube-like mouthparts. Like many parasites, they tend to specialize in specific groups of plants: knowing what plants they feed on can help greatly in identification.
Life Cycle
While there's some variation, they start as free-moving crawlers, with the females becoming less mobile as they mature. In most groups, the females attach to a single spot and lose legs, antennae, etc., so that they begin to look more like some kind of growth than an insect. Both mobile and non-mobile types develop thick protective layers of wax or other inert substances, often in elaborate shapes- so it's hard to see the actual insect underneath.
The adult males are rarely seen (or at least rarely noticed), but look somewhat like winged aphids:
The female lays her eggs where she is (often in a large sac hidden under her own protective covering), and the crawlers hatch to move on to new feeding sites.
Remarks
Most gain the protection of ants by secreting a sugary substance called honeydew.
Normally only a minor pest (mostly because they spread diseases), but non-native species that lack natural enemies can build up to devastating numbers. Some of the earliest and most dramatically successful uses of biological control have been against this group.
See Also
Whiteflies undergo a similar legless, attached stage as nymphs, but then both males and females develop into winged adults.
Internet References
Zootaxa. Phylogeny and Classification.
