The Torah prohibits transporting anything on the Sabbath via a public highway, or transferring anything between a public highway and a private residential area. Although we colloquially call this prohibited activity “carrying”, it includes all manner of transferring.
King Solomon (and later sages) extended this prohibition to include transporting within and between many other kinds of areas. However, they also provided a mechanism for a community to come together to create a domain in which those rabbinic prohibitions don’t apply.
An important prerequisite for establishing such a domain is that it must be physically bounded by partitions, such as fences, hedges, walls, etc. The partitions can sometimes be more symbolic than substantive. For example, existing utility wires can often be used (with permission of the utility companies of course) as a symbolic “lintel.” By adding a vertical post (called a “lechi”) directly under each end of the wire, a symbolic “doorway” can be made. The resulting “form of a doorway” can be a valid part of the boundary. This is usually the most cost effective way to bound a large area. Building valid partitions around a city or neighborhood is a significant construction project, but most areas with large Jewish populations do so.
"Eruv" means 'merging'. Technically, an eruv is the legal procedure that temporarily merges many private domains each Sabbath, for which the physical boundary is a prerequisite. But the boundary itself is colloquially called an eruv (pronounced “ei-roov”).
An eruv is particularly important to young observant families, since it permits the whole family to come to synagogue together. (The prohibition against transporting includes carrying young children who cannot carry themselves.) Without an eruv, one parent stays home with the young children on Sabbaths (and also on Yom Kippur), or else a baby-sitter must be hired. Thus many families will not consider moving to a neighborhood without an eruv, even if housing costs are reasonable and the synagogue is a very special place. This is particularly so if other neighborhoods in the same job market do have an eruv.
An eruv is a convenience that enhances the enjoyment of the Sabbath for everyone. For example: a Jew can bring wine to a friend's house for kiddush, take printed Divrei Torah home from shul on Saturday mornings, carry a tallis bag to and from services, or bring extra food to the shul for shalosh seudos (Third Meal). An eruv also makes a statement. It says that this is a Jewish community in which the Sabbath is observed.
The telephone, power, and cable TV companies sometimes move the wires on which the boundary depends. So late Thursday, two professional checkers inspect the entire boundary. An electrician is on retainer to make any needed repairs on Friday. The current status of the eruv is reported to the public on the Eruv Status Page of our website, on the website of The Greater Boston Eruv, and by a recorded phone message, usually updated between noon and 2 PM each Friday.