Cellar spiders belong to the group of spiders known as the Pholcids, and are among the most numerous in the world. They also go by several different names depending on where you are in the world, often being referred to as daddy longlegs in the United States for example. They are actually only one of three species known as daddy longlegs, with the harvestmen and crane fly being the other two.
The crane fly is called the daddy long legs in the United Kingdom, and the Harvestman goes by the same name in Australia. The cellar spider having taken the name in the United States, although in actual fact the three species aren't very similar at all. The crane fly is a large clumsy flying insect that looks like a giant mosquito although it doesn't actually have any mouthparts and cant feed. The Harvestman is an arachnid, although not a spider, and doesn't make a web either.
The confusion probably happened because all three species have long thin legs and was at some point confused as being each other by travellers. But knowing the difference between the three species is important when studying cellar spiders. Cellar spiders are as with all spiders exclusively predatory, and are commonly encountered by man because they live close by. Their bite however is harmless, and they are shy to the point of retreat when encountering humans.
Cellar spiders are usually found inside the home, and in fact usually live exclusively in human dwellings. In appearance they have long bodies compared to many spiders, and have very long thin legs, which are up to three times the length of their bodies. It is because of their frailty that they probably prefer secluded areas such as houses rather than the outdoors. They vary greatly in size, but at their largest can reach sizes of 40mm (leg span).
Typically cellar spiders make large webs that hang in messy clumps, trapping their prey through it being tangled rather than stuck. That is to say that their webs aren't sticky as with many web making species, but instead are finely woven making them easy to get caught in. This is why they often hang both horizontally and vertically and don't appear to conform to regular patterns.
Another trait of Pholcids is that many of them target other spider species as their prey, including the dangerous Red back, Hobo spider and Huntsman spiders. In fact they are often thought to be beneficial to households because they are harmless to humans and pets, but will keep more dangerous spider populations down.
As well as being good for keepings pests away, cellar spiders are also unlikely to disturb humans by running across the ground, as they generally stay in their webs and tend not to roam onto the ground very often. They also prefer to live close to their mates rather than only briefly meeting as most arachnids do.
Curiously, cellar spiders aren't affected by other spider populations very much, other than giant house spiders found in the United Kingdom and parts of Europe. Typically spider populations depend largely on competition from other species in the area. So for example populations of hunting spiders such as wolf spiders, often lower the populations of web making spiders in the immediate area. This isn't as prevalent in outdoor areas because of the higher numbers of flying prey, but indoors this effect can be numerically significant.
The most common prey for cellar spiders is usually small flies, mosquitoes and other spiders, although tat they can they will eat anything that they can easily overpower. They are also known to let larger, more powerful prey escape rather than try to tackle it. Things such as wasps, hornets etc, will often then eventually escape of their own accord.