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.