If it's big, and on a conifer, it's probably Cinara. To identify further, it's usually necessary to identify the host plant, and consider the geographic range of different species.
The genus is widespread, but individual species are often limited by the range of their hosts.
Given their host preferences, usually in coniferous forests. Some species, however, feed on junipers and pinyon pines found in drier, more open habitats.
Feeds on the sap of conifers. Many species are very specific as to host plant.
Tends to form colonies on individual trees. They secrete honeydew, which is eaten by ants and wasps and provides the substrate for sooty mold fungus. May cause some stunting or even death on small or already-stressed hosts, but generally not a serious threat.
They are, however, a problem for Christmas tree growers: customers don't like large, conspicuous aphids in their homes, especially since they tend to abandon the tree as it starts to dry out.