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Gymnosperms were the first taxa of seed plants to evolve. They appeared in the fossil record in the late Paleozoic (approximately 300 million years ago) and dominated dry terrestrial landscapes soon thereafter. The evolution of seeds liberated these plants from the requirement for liquid water for reproduction and, in large part, explains the ability of gymnosperms to capitalize on previously unexploited dry habitats. Over time gymnosperms evolved additional features that enabled them to tolerate dry conditions, including the ability to limit transpirational water loss and conduct water up tall stems.

Cones are the reproductive structures for all gymnosperms. They produce seeds which are not enveloped in a fruit (in fact, the name gymnosperm derives from the Greek roots meaning "naked [or fruitless] seed"). Alongside the evolution of seeds came the evolution of pollen as a means of transporting male gametes to the female gametes, which are located deep within cones, so that sexual reproduction can be completed. Typically, gymnosperms employ wind pollination (there are exceptions), a comparatively inefficient mechanism that necessitates the production of massive amounts of pollen.

The term "gymnosperm" is often used interchangeably with "conifer", however conifers are only one subgroup of gymnosperms (albeit the most specious). Other subgroups contain relatively few extant members and include morphologically diverse forms such as Ginko biloba, Gnetum, Ephedra, Welwitschia, and cycads. Conifers are perhaps the most conspicuous members of this group (certainly in Maine), comprising over 550 of the 720 living species of gymnosperms. Most conifers possess scale-like (i.e. junipers) or needle-like leaves that persist on the plant for more than one year. The shape, appearance, and number of needles in a bundle are characteristics that are often employed to identify individual coniferous species. The size and appearance of cones, as well as their location on the plant, are additional characteristics that are useful for this purpose.