Kingdom Plantae

At the top of Carolus Linnaeus's original system of classification were two kingdoms, representing plants and animals. The plant kingdom has been revised a bit since then, but there are still distinguishing characteristics that cause biologists to group all plant species together.
• Plant cells contain chloroplasts filled with chlorophyll, which they use to make their own food via photosynthesis.
• Plant cells have cellulose cell walls
• Most plants require being anchored to a surface of some sort: the ground, the sea bottom, or another plant.
• Plants reproduce sexually, but in a peculiar way called "alternation of generations" (Follow the link, because I put a lot of work into that one).
Plants appeared during the Ordovician Period (after animals), and were the first living organisms to colonize the land. Paleozoic forests were dominated by giant horsetails; ferns evolved into cycads, which evolved into conifers, and finally, by the Cretaceous, the flowering plants which dominate the kingdom today.
Plants have been classified morphologically, cladistically, and even by height. Because of this, primary divisions (preferred to "phylum" in modern classification) of Plantae can have several names.
The most closely related organisms to plants are members of the Kingdom Chromista, or "algae", particularly the Chlorophyta ("green algae") and Charophyta (stoneworts). Many Chromista are auotrophic via photosynthesis, can reproduce sexually, and some even alternate generations as plants do. In other types of Chromista, cells contain a different type of chlorophyll and do not store energy as starch. Because the Charophyta and Chlorophyta use chlorophyll a and b, but are structurally and genetically similar to other algae, it is difficult to place these two groups in a kingdom. Thus, some plant classifications contain the Charophyta and Chlorophyta. But many classifications do not include them, and restrict kingdom Plantae to land plants.
Since the gametophyte-dominant plants and the sporophyte-dominant plants probably evolved from different algal ancestors, "Plantae" is probably polyphyletic unless the two groups above are included.
• Non-vascular plants are (24,000 species) are dominated by the gametophyte generation. The "plant" you see most of the time is the gametophyte, and the sporophyte ("fruiting body") is usually parasitic on the gametophyte which produced it. After dispersing their spores, moss and liverwort sporophytes die off.
o Division Anthocerotophyta (hornworts)
o Division Bryophyta or Musci (mosses). "Bryophyta" often refers to all three gametophyte-dominant groups.
o Division Hepaticophyta (liverworts)
• Tracheophytes, or "vascular plants", are dominated by the sporophyte generation.
o Division Rhyniophyta (EXTINCT, Silurian - Devonian)
o Division Zosterophyllophyta (EXTINCT, Devonian)
o Division Trimerophyta (EXTINCT, Devonian) evolved the first leaves and gave rise to the more complex spore- and seed-bearing plants.
o Spore-bearing
 Division Equisetophyta aka Sphenophyta (one genus. Equisetum, horsetails, 40 species)
 Division Lycopodiophyta aka Lycophyta (club moss, quillwort, 1,000 species)
 Division Polypodiophyta aka Pteridophyta aka Filicinophyta (ferns, 12,000 species)
 Division Psilotophyta (whisk ferns, 3 species)
o In seed-bearing plants (Spermatophyta), the entire haploid part of the cycle takes place in the gametangia, which grow on the sporophytes. Male gametophytes appear as small particles we call "pollen", which are carried into the female gametangia of other plants by the wind, or by animals (usually insects) which the plant has evolved to attract. After fertilization, the zygote is encapsulated with a food reserve into a "seed" which grows the next sporophyte.
 Gymnosperms are grouped by some into a single division, Pinophyta. They are considered an arbitrary grouping by others.
 Division Pteridospermophyta ("seed ferns") (EXTINCT, Devonian - Triassic)
 Division Coniferophyta (conifers, 600 species)
 Division Bennettitophyta (cycadeoids, Triassic through Cretaceous)
 Division Cycadophyta (cycads)
 Division Ginkgophyta (one extant species, Ginkgo biloba)
 Division Gnetophyta (three families)
 Division Magnoliophyta (Angiosperms: flowering plants, 250,000+ species). The name Anthophyta has recently arisen to contain the Magnoliophyta and the similar Bennettitophyta, Gnetophya, and Gingkophyta.