As per Marvel.com, in the comics, we see the web-shooters use web fluid, described as:
a shear-thinning liquid (virtually solid until a
shearing force is applied to it, rendering it fluid) whose exact
formula is as yet unknown, but is related to nylon. On contact with
air, the long-chain polymer knits and forms an extremely tough,
flexible fiber with extraordinary adhesive properties. The web fluid's
adhesive quality diminishes rapidly with exposure to air. (Where it
does not make contact with air, such as the attachment disk of the
web-shooter, it remains very adhesive.) After about one to two hours,
certain imbibed esters cause the solid form of the web fluid to
dissolve into a powder. Because the fluid almost instantly sublimates
from solid to liquid when under shear pressure, and is not adhesive in
its anaerobic liquid/solid phase transition point, there is no
clogging of the web-shooter's parts.
which is first seen in Amazing Fantasy #15 (1962). So, in the comics, it's more of a nylon material.
In the Spiderman trilogy (2002-7), this ability is described as:
Organic Webbing Generation: In the Sam Raimi movie trilogy, Spider-Man
was also gifted with the ability to organically produce his own silk
webbing from glands within his forearms, limited by his body's health
and nutrition. These organic webs have many of the same properties as
a spider. Thanks to its similar properties, it appears Spider-Man can
utilize his organic webbing in any way he could with the artificial
webbing from the comics.
Conversely, in the movies it seems to be more like spider's silk. So the answer is, it depends which source you're focusing on.