When Peter Parker was bitten by a radioactive spider, he found he had the proportional speed, strength and agility of a spider, as well as the ability to stick to walls. This inspired him to create his signature weapon and most used tool of all, his web shooters.
Fitting around each wrist, Spider-Man's web shooters are easily concealed under his costume like a magic trick.
The trigger lies flat in his palm and requires a quick double tap of his middle and ring finger to activate. This keeps him from accidentally discharging his web when making a fist or holding an object. Not only does it need a double tap, Spider-Man's web shooter also needs a strong amount of pressure to activate, beyond what a normal human can produce. Each shooter has an adjustable nozzle to allow for strong strands for web-slinging or wide dispersal for web nets, shields, cushions, and more. The shooter’s nozzle reacts to minute flicks of Spider-Man’s wrist to adjust the spray.
Each shooter has a compartment for a small cartridge of concentrated web fluid which he had developed at home using a chemical set he had. He keeps spare cartridges in a belt under his costume for easy access. In later times, he improved them by installing a rotating carousel that automatically reloads a web cartridge should the current cartridge inside the chamber of the web shooter runs out of web fluid. A steel pin breaks the cartridge, ready to shoot web for web-swinging or ensnaring villains. The web shooter is also prone to malfunction, thus Peter needs to have constant maintenance to prevent his webbing from exploding and entangling himself.
Later added was a voice activated function. This allows him to shoot continuous blasts of webbing in different directions and forms. It is unknown what else the function can do at the moment.
His clone, Ben Reilly, uses a modified web shooter that is much the same as Spider-Man's but the difference is that it is worn outside his costume. Peter's daughter from an alternate future, May "Mayday" Parker, aka Spider-Girl, used the same web shooters her "Uncle" used. Back when they were married, Peter gave Mary Jane a modified web shooter disguised as a bracelet that she can use for self-defense as a Valentine's Day gift.
When Spider-Man taps into his web shooter trigger, the web fluid is shot from a small nozzle. When the web fluid comes in contact with the air, it solidifies into a rope-like material that Spider-Man uses to web-swing across the New York skyline. If Spider-Man taps the trigger hard enough, the nozzle widens its opening, making the web coming out of the web shooter wider. The webbing produced by these cartridges is ultra-strong, but dissolves in about one hour’s time, this is why New York City isn’t covered in discarded webs. Spider-Man has improved his webbing formula over the years as he gained access to better labs and chemicals, and he has also developed variations on his web fluid formula for use against specific villains such as when he fights Venom he has an extremely strong and sticky webbing, insulated webbing against Electro, acidic webbing to melt through Rhino's tough hide and flame-retardant webbing to fight fires or fire elemental enemies.
When Spider-Man fought the Blob, he intentionally took one of his web cartridges and threw it towards the villain, the cartridge broke, exposing the web fluid in the air, overwhelming the Blob into a mesh of webbing, leaving him stuck for the police to arrest him. Ben Reilly, aka the Scarlet Spider, developed his own special web called Impact Webbing, a ball of web that engulfs a villain into a cocoon, and Stingers, darts that have a sedative to tranquilize anyone hit by it.
In the Sam Raimi Spider-Man movies (Spider-Man, Spider-Man 2, and Spider-Man 3), Peter had organic webbing generated by glands in his forearms. This was controversial with fans at the time, but proved popular with general audiences.
To capitalize on this, Marvel decided to introduce the organic web-shooters into the comics during the Avengers Disassembled crossover. In the story arc, Peter evolved biological web shooters located in his forearms. He developed spinnerets under his forearm that can trigger in a similar means to his original web shooter, but without the burden of constant repair of the device and costly web fluids. The strength and elasticity of the organic webbing is unknown, but it is speculated to be much stronger than the synthetic and is said to depend on Spider-Man's current health and nutrition. Unlike the synthetic webbing, the organic webbing last for a week until it dissolves. But ever since One More Day, Peter has lost the ability of organic webbing and is therefore using his web shooters again.
In Other Media
The web-shooters appear in The Amazing Spider-Man and The Amazing Spider-Man 2. In the first movie, it is shown that the webbing was developed from a bio-cable sold by OsCorp, which was made from the webs of the genetically engineered spiders that bit Peter. Peter jury rigs his first pair of web-shooters from a pair of wristwatches, but uses a more professional set in the sequel.