"If you're going to stay, you will only hurt bad people."Symbiotes are a alien race who landed on Earth and is taken by the Life Foundation to experiment on Humans, Before Eddie Brock bonds with one of the Symbiote becoming Venom.
"The way I see it... we can do whatever we want. Do we have a deal?" ― Eddie Brock and Venom[source]
Symbiotes are able to transfer themselves from host to host upon physical contact with them, passing into their flesh as if intangible and taking full control of their bodies. While in their bodies, these creatures are able to enhance their physical abilities to inhuman levels, allowing them to overpower other humans with ease and heal from severe injuries such as broken bones and damaged tissue in seconds. While it is capable of seemingly suppressing a host's mind and use their bodies as if it were its own, as Riot did while possessing various people before reaching Carlton Drake if their host is accepting of them, it is able to almost meld minds with them. Additionally, while physically merged with their host, a symbiote is able to perceive its host's thoughts, feelings and memories and communicate to them in the form of auditory and sometimes visual hallucinations, causing a host to hear the symbiote speaking in their minds. With their body and minds united in this manner, both the symbiote and its host can work and think collectively while sharing the same body.
The abilities it can use while bonded to its host, including its ability to manifest constructs out of its own mass and enveloping its host to form a body for itself, is fueled by how compatible it is with its host. The process of Symbiotes bonding was deemed similar to organ transplants in humans. This can cause a Symbiote to become lethal towards their host's if they are not compatible, which in turn could lead for the Symbiote itself to die as well.
Additionally, even when finding an ideal match to bond with, Symbiotes still have a degree of toxicity to their host's body, as its hunger for organic matter leads it to subsist partially on their organs. Additionally, they cause their host's metabolism to rise tremendously, resulting in not only a voracious appetite. In order to combat this, while manifesting its own body over their hosts, Symbiotes are prone to eat other living creatures to avoid having to consume or otherwise damage their host. (Some, like Eddie Brock, are willing to let the Symbiote eat others if they're bad guys.)
To be added