From Wikipedia on spider webs:
There are a few types of spider web found in the wild; and many spiders are classified by the webs they weave. Different types of spider webs include:
Spiders have several spinneret glands located at their abdomen which produce the silken thread. Each gland produces a thread for a special purpose. Seven different gland types have currently been identified, although each species of spider will possess only a few of these types, never all seven at once.
Normally a spider has three pairs of spinnerets, but there are spiders with just one pair or as many as four pairs of spinnerets, with each spinneret having its own function.
During the process of making a web the spider will use its own body for measurements, a very practical and ergonomic design feature of any web. This will allow the spider to move quickly around its own web with very few faults.
It will start with the most difficult part of construction, the first thread. The spider effectively uses the wind to carry its initial adhesive thread. With some luck the silk is released from its spinners and carried by the wind to a suitable adherable surface. When it sticks to a surface the spider will carefully walk over the thread and strengthen it with a second thread. This process is repeated until the primary thread is strong enough to support the rest of the netting.
After strengthening the first thread the spider will continue to make a Y shaped netting. The first three radials of the web are now constructed. More radials are added making sure that the distance between each radial is small enough to cross. This means that the number of radials in a web directly depends on the size of the spider plus the size of the web.
After the radials are complete the spider will fortify the center of the web with about five circular threads. Then a spiral of non-sticky, evenly spaced, circular threads are made for the spider to easily move around its own web during construction. The spider then, beginning from the outside in, will methodically create the adhesive spiral threads. It will utilize the initial radiating lines as well as the non-sticky spirals as guide lines. The spaces between each spiral will be directly proportional to the distance from the tip of its back legs to its spinners. This is one way the spider will use its own body as a measuring/spacing device. While the sticky spirals are formed the non-adhesive spirals are removed as there is no need for them anymore.
After the spider has completed its web it will chew off the initial three center spiral threads then sit and wait. If the web is broken without any structural damage during the construction the spider does not make any initial attempts to rectify the problem.
Webs allow a spider to catch prey without having to expend energy by running it down. Thus it is an efficient method of gathering food. However, constructing the web is in itself an energetically costly process due to the large amount of protein required, in the form of silk. In addition, after a time the silk will lose its stickiness and thus become inefficient at capturing prey. It is not uncommon for spiders to eat their own web daily to recoup some of the energy used in spinning. The silk proteins are thus 'recycled'.