The female spider lays her eggs than coves them with silk making an egg sac. What she does next depends on the species. Some spiders secure the eggs sac in a suitable place and leave it, others secure it and stick around to guard it and some open it up and tend to the eggs, some attach the eggs sac to their bodies and haul it around with them, opening to tend and sun the eggs. Egg sacs can be used to identify the type of spider. Below are photographs of just few.
Above is a wolf spider carrying her egg sac attached to her. Wolf spiders do not build webs, and are very attentive mothers. She will open the egg sac from time to time and move the eggs around to check they are fine, and take them on to a warm, sunny stone on fine days to bask in the sun. And even after the spiderlings have hatched and left the sac she will carry them around on her back for a few days.
Below is a photograph of an egg sac containing spiderlings. This sac was on the underside of a firewood log.
Below is a very common spider fund in most gardens, she is Enoplognatha ovata (Theridion ovatum), the Common candy-stripe spider. You do not normally see her with her egg sac though. I disturbed this one while gardening. I cut back the leaf where her sac was kept, see the photograph below.
She says nearby guarding the eggs. To make up for my disturbance I put both her and leaf into my greenhouse, and pretty soon I had a load of spiders keeping the place pest free.
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Below is the egg sac of a pirate spider. They are placed at the end of a long, springy stalk and left hanging making it difficult for predators, also the silk used for the sac is soft, but the springy outside silk is like wire, so offers more protection to the eggs.