In Amazing Spider-Man Annual #1 (1964), it was stated that Spidey could expel his webbing in three different ways by adjusting the nozzles on his web-shooters.
By adjusting the nozzle of his web-shooter in one easy motion, Spidey can eject his web fluid in any one of three different ways...
As a thin, incredibly strong line...
As a fine, quick-spreading spray...
Or as a thick, tremendously adhesive liquid...!
By Amazing Spider-Man Annual #4 (1967), he'd apparently modified his web-shooters so that he could vary the output based on how many times, and for how long, he depressed the triggers on his palms.
The amazing web-fluid can gush out in many different forms, depending upon how long Spidey's finger remains upon the sensitive control button...
A short tap releases a thin, cable-like strand, recommended for swinging... or stringing guitars!
A longer period of pressure releases more fluid, forming an icky, echhy, blochhy, sticky blob... useful in pasting a foe against a wall... like!
A series of brisk, staccato taps releases multiple cable shapes which can be formed into a decorative netting pattern!
A similar explanation was given in Spider-Man: The Ultimate Guide (2007).
Peter designed his web-shooters so that he wouldn't accidentally fire them every time he made a fist. Resting in the palm of his hand, the trigger works just like a computer's mouse. He must tap twice in rapid succession to release his webbing.
Peter has improved upon his initial design. He now switches between different forms of webbing by the way he taps his trigger. With a short second tap he releases a thin cablelike strand that is perfect for web-swinging. A longer second tap increases the strand's thickness for additional support. If Spidey prolongs the pressure on the trigger, web fluid squirts out in the form of an adhesive liquid that can paste a foe against a wall. A series of brisk taps discharges many thin strands that form a fine spray of webbing, perfect for blinding an opponent.
Spider-Man's entry in the Official Handbook of the Marvel Universe A-Z #11 (2009) provides detailed diagrams of both the mechanical and organic web-shooters, and states the following:
The effect of the very small turbine pump vanes is to compress (shear) the web fluid and then force it, under pressure, through the spinneret holes, which cold-draws it (stretches it: the process wherein nylon gains a four-fold increase in tensile strength), then extrudes it through the air where it solidifies. As the web fluid exits the spinneret holes, it is attracted to itself electrostatically and thus can form complex shapes. The spinneret holes have three sets of adjustable, staggered openings around the turbine that permit a single webline, a more complex, spun webline, and a thick stream.