Web-Shooters were created by Peter Parker to fire spider webs.
History[]
After the spider passed on the knowledge of what chemicals to use to create webbing. Peter then created a web fluid that he placed in a small cartridge and a pair of Web-Shooters. The cartridge would be placed in the Web-Shooter and would fire a web when Spider-Man would press the trigger with his two middle fingers.
According to Spider-Man the webbing only lasts for about an hour before it starts to dissolve.
The webbing could also be fired two different ways from the web-shooters. The first way is that the web could be fired as a thin web line and the second way was as widespread.
The webbing could be formed into various objects such as a net, a parachute, a swing, or a bag that Peter used to store his clothes in.
Peter Parker later created a special type of web fluid to use against Hydro-Man. This new type of web fluid hardened when exposed to water. Spider-Man would use this to harden Hydro-Man's liquid body so that he could not move.
Green Goblin was so strong that he was able to crush Spider-Man's Web-Shooter's by simply squeezing his wrists.
In an alternate reality, Scarlet Spider designed what he called impact webbing. This special type of webbing could be fired as a big ball of webbing and acts as a non-lethal projectile.
Non-canon history[]
In Adventures of Spider-Man #5 a scientist named Doctor Pealé creates a virus that can break down and eat chemical compounds. Spider-Man adds this virus to his web fluid to fight Rhino. When Spider-Man fights Rhino, he sprays him with his webbing and the added chemical inside the web fluid dissolves Rhino's suit.