The Basics
What is a carnivorous plant?
Carnivorous plants must possess three basic traits in order to be considered truly carnivorous:
1. They must have some mechanism for attracting and/or capturing prey.
2. They must possess some means of digesting the prey captured, either via their own enzymes or through associations with other organisms.
3. The nutrients from captured and digested prey must provide some significant benefit to the plant.
Many plants exist that capture insects but do not digest them or directly benefit from them. Traits such as sticky leaves or water in the axils of leaves act as defense strategies in order to prevent the plants from being damaged by pests. Tomatoes are a familiar example, with their leaves and stems covered in sticky hairs (or trichomes) that capture small insects such as aphids and fungus gnats, and wild teasel has leaves that wrap around the stem and capture water, preventing crawling insects from reaching newer leaves higher up on the stem.
Some plants are clearly adapted for the capture of insects, such as species in the genera Darlingtonia, Heliamphora, and Roridula, but unlike other carnivorous plants do not possess their own enzymes to break down prey and in the case of Roridula, the capture method does not permit digestive enzymes to be used). Some might consider these to be "paracarnivorous" plants, those with some traits of carnivory and perhaps benefiting from captured prey but not truly carnivorous because they cannot "finish the job" themselves. However, the unique trapping mechanisms are also paired with glands capable of absorbing nutrients that are broken down by symbiotic organisms such as midge and mosquito larvae in the pitcher plants or assassin bugs living on Roridula, such that the plant may not do the digesting, but it still receives a benefit from captured prey. This may even save energy, the plant not having to spend it on producing enzymes but instead merely playing host to another organism (which in turn benefits from the plant by having a home and free food).
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How do carnivorous plants capture prey?
Carnivorous plants are generally divided into a handful of different categories via the method of prey capture: pitfall, flypaper, bear or snap traps, suction, and lobster pot traps.
1. Pitfall traps: These use gravity as the primary means of capturing prey, enticing the organisms to to opening of the trap with promises of nectar or other food and luring them in to the point where they reach a slippery zone that can't be gripped. From there, they fall into a pool of liquid at the bottom of the trap, drowning and being digested like an open stomach.
2. Flypaper traps: Leaves covered in sticky glands or hairs that organisms crawl or land on and become glued to, incapable of extracting themselves and their struggles often miring them further in whatever concoction is used to snare them. Most carnivores use some form of mucilaginous secretion (think snot or spit in texture), but Roridula use a modified sap, which is even more powerfully sticky but prevents the diffusion of enzymes.
3. Snap traps: Found in only two species in two separate genera, this is the most familiar trap type to many. Two halves of a clamshell-shaped leaf sit open in wait, the inside secreting nectar or other attractants and lined with trigger hairs that prey brushes against. Once the triggers are touched, the trap snaps shut, closing the prey inside and sealing before digesting whatever was captured.
4. Suction traps: Unique to the bladderwort genus (though suction is theorized as possibly present in their relatives the corkscrew plants), these traps are designed like little balloons, the space inside them emptied by specialized glands and producing a negative pressure waiting to be released by triggers surrounding the mouth or "trapdoor" of the trap. Organisms passing by the triggers cause the door to swing open inward and the pressure to suck the prey in, before the trap closes again and begins digestion and the removal of excess water or air to reset the trap. These are among the fastest moving organs in the plant kingdom, some species recorded as triggering and closing again within 1/2,000 of a second (for reference, blinking your eye takes on average 1/3 of a second).
5. Lobster Pot traps: The corkscrew plants are the primary genus using this technique, though lobster pot designs are seen in some species of other genera as well (covered below and in species accounts). Prey entering the trap opening typically encounter a series of inward-pointing hairs, often intermeshed, that permit travel only in one direction. Once inside the first layer of hairs the prey cannot back out, either physically prevented by interlocking hairs or speared by the sharp tips, so it has no choice but to move forward until it eventually reaches the digestive zone.
Some species of carnivorous plants utilize a combination of techniques in their trapping style. The most common is a modification of the pitfall trap to contain elements of lobster pot traps, seen in Sarracenia, Darlingtonia, and Nepenthes in particular. Though gravity often still does a lot of the job, constricted pitcher openings or intermeshed hairs within the trap guide the insects in a one-way path toward death by drowning. Pitfall-flypaper traps also exist in some plants like Drosophyllum, to be covered more in detail in the species accounts.
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Where do Carnivorous Plants Live?
Just about everywhere! Carnivorous plants are adapted to survive in low-nutrient environments, places where the soil can't or simply doesn't contain the nutrient levels other plants need to survive. Some, even, are adapted to soils that are toxic to other species, like the high-metal ultramafic soils of the Pacific Northwest or high mountain slopes of Southeast Asia. Many are associated with being bog or swamp plants, but other low-nutrient environments include sandy savannahs, oligotrophic waterways, pine barrens, fens, and believe it or not, many tropical rainforest and savannah soils where nearly everything is in competition for what little nutrients are locked almost entirely in the first few inches of leaf litter on the ground.
As for where in the world, carnivorous plants are found on every continent except barren Antarctica, and even every state in the US and nearly every country across the globe. It's a fair chance that carnivorous plants exist little more than a couple hours away from wherever you might live.
That doesn't mean of course that a quick drive outside is going to land you in carnivore territory. The habitats they enjoy are often sparse, small singular patches hiding in the backwoods or remnants of what used to be far larger territories. Many species are highly endangered, threatened by habitat loss, poaching, degradation and fragmentation of habitat, and more, so even those species that are more abundant may be protected on preserves with limited travel access. Those not on preserves may also have their locations hidden by caring enthusiasts (myself included), and location data is rarely given out. If you want to see carnivorous plants in their habitat, your best chance is to find preserves and parks with trails leading past the plants, places where they are often well-cared for and watched over. These may include:
Darlingtonia Wayside in Oregon
Big Thicket Preserve in Texas
Weeks Bay National Estuarine Research Reserve in Alabama
...and many others. Though I am limited more to known sites in the US, there are parks and preserves with carnivores in many other countries as well with spectacular plants.
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Though this site will continue to be expanded and edited to include as much as possible, there are many subjects that one person simply cannot cover with a truly thorough scope. For more information, including fun history, photographs, cultivation guides, etc., make sure to check out the many books that exist on carnivorous plants, such as:
Growing Carnivorous Plants by Barry Rice
The Curious World of Carnivorous Plants by Barthlott et al.
Carnivorous Plants of the United States and Canada by Donald Schnell
The Savage Garden (THE CARNIVORE GROWER'S BIBLE) by Peter D'Amato
The Redfern Natural History Publications by Stewart McPherson and others
...and many, many more!
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Also , make sure to to follow the links below for sites with similar information. The Carnivorous Plant FAQ is a great site for beginners with a lot of basic info, the CP Photo Finder a fantastic resource for images of all sorts of species and hybrids or cultivars (and the source of many images for the species in the Database that I have not yet owned), and more.