Venom is a super villian/anti hero/hero made by marvel comics. Venom and its kind are born on the planet Klyntar. Venom was hatched from an egg, the Klyntar are supposed to be protectors of the cosmos. Unfortunately Venom's first host was an evil creature and used the Venom symbiote to massacre everything on his home world. Due to Venom bonding to this creature, the Venom symbiote was twisted into a predatory and hate-filled being addicted to rage. His brethren decided to lock the Venom up, Because they thought it was too much of a risk to keep around.
The first human that bonded to Venom was Wade Wilson aka Deadpool. This happened during the events of BattleWorld. When it bonded to Wade he noticed that it was starting to effect his mind, so he put it back in its cell. After that Spider-Man came to Battleworld and found the cell were Venom was placed. The symbiote was let out of its cage and bonded to Spider-Man forming a new black and white costume. Spider-Man later realized that the suit could form into street clothes and also could shoot its own strong webbing.
When Spider-Man got back from Battleworld, he wore the Venom symbiote for a long time. Then after a while Spidey wanted to know what exactly this suit was, so he went to Mr. Fantastic for help. Thanks to Mr. Fantastic Spidey now knows that the black suit is a sentient alien. Mr. Fantastic also found that the Venom symbiote is vulnerable to piercing loud sounds and extreme heat. By using these vulnerability's they were able to get the Venom symbiote off Spider-Man and into a containment module. Sadly Venom escaped and hid in Spider-Mans closet.
After a huge battle Spider-Man went into his closet for a spare suit when the Venom symbiote forcefully bonded to Spider-Man. In a frantic attempt to get the Venom symbiote off him, Spider-Man fled to a bell tower. When the bell rang Spidey was able to get the Venom simbiote off him. With the last remaining strength Venom had he ran away through the cracks of the floor. Spider-Mans rejection left Venom bitter toward him, a trait it would share with most of its future hosts.
Now that's just the beginning of its story. After what I just told you he bonds to a lot more people such as Eddie Brock, Mac Gargan, and Flash Thompson. Which maybe I will right about another time. Because if I try writing about all of them now we will be here for ever.