One of the 39 categories of creative work is "transferring from domain to domain" (also called "carrying").

Basically this means that we do not carry any object — anything from a piano to a house key — from a "private domain" (an enclosed area such as a house or a fenced yard) out into a "public domain" (e.g., a city street), or vice versa. We also do not carry an object in a "public domain" for a distance of more than four cubits (about 6 feet).

An eiruv is a mechanism which enables carrying on Shabbat between homes and apartments within a designated area. The eiruv has two basic components: a) a physical enclosure which makes the area inside it the equivalent of a "private domain"; b) a "common meal" that symbolically knits all Jewish households in the area into a single household. An eiruv can encompass a group of houses, or even, under certain circumstances, an entire neighborhood or city. Because the laws of eiruv are very complex, consult your rabbi before making use of one.