The term "biotoxin" is sometimes used to explicitly confirm the biological origin as opposed to environmental or anthropogenic origins. Biotoxins can be classified by their mechanism of delivery as poisons (passively transferred via ingestion, inhalation, or absorption across the skin), toxungens (actively transferred to the target's surface by spitting, spraying, or smearing), or venoms (delivered through a wound generated by a bite, sting, or other such action). They can also be classified by their source, such as fungal biotoxins, microbial toxins, plant biotoxins, or animal biotoxins.
Toxins produced by microorganisms are important virulence determinants responsible for microbial pathogenicity and/or evasion of the host immune response.
Biotoxins vary greatly in purpose and mechanism, and can be highly complex (the venom of the cone snail can contain over 100 unique peptides, which target specific nerve channels or receptors).
Biotoxins in nature have two primary functions:
* Predation, such as in the spider, snake, scorpion, jellyfish, and wasp
* Defense as in the bee, ant, termite, honey bee, wasp,  poison dart frog and plants producing toxins
** The toxins used as defense in species among the poison dart frog can also be used for medicinal purposes
Some of the more well known types of biotoxins include:
* Cyanotoxins, produced by cyanobacteria
* Dinotoxins, produced by dinoflagellates
* Necrotoxins cause necrosis (i.e., death) in the cells they encounter. Necrotoxins spread through the bloodstream. In humans, skin and muscle tissues are most sensitive to necrotoxins. Organisms that possess necrotoxins include:
** The brown recluse or "fiddle back" spider
** Most rattlesnakes and vipers produce phospholipase and various trypsin-like serine proteases
** Puff adder
** Necrotizing fasciitis (caused by the "flesh eating" bacterium Streptococcus pyogenes) – produces a pore forming toxin
* Neurotoxins primarily affect the nervous systems of animals. The group neurotoxins generally consists of ion channel toxins that disrupt ion channel conductance. Organisms that possess neurotoxins include:
** The black widow spider.
** Most scorpions
** The box jellyfish
** Elapid snakes
** The cone snail
** The Blue-ringed octopus
** Venomous fish
** Frogs
** Palythoa coral
** Various different types of algae, cyanobacteria and dinoflagellates
* Myotoxins are small, basic peptides found in snake and lizard venoms, They cause muscle tissue damage by a non-enzymatic receptor based mechanism. Organisms that possess myotoxins include:
** rattlesnakes
** Mexican beaded lizard
* Cytotoxins are toxic at the level of individual cells, either in a non-specific fashion or only in certain types of living cells:
** Ricin, from castor beans
** Apitoxin, from honey bees
** T-2 mycotoxin, from certain toxic mushrooms
** Cardiotoxin III, from Chinese cobra
** Hemotoxin, from vipers