Answer:
   
   The Torah prohibits carrying on Shabbat between a public domain and a
   private domain or for more than 4 cubits in a public domain.. However,
   the Torah permits carrying within an enclosed "private" area. Public
   domains are typically non-residential areas including streets,
   thoroughfares, plazas ("open areas"), highways, etc. Private domains
   are residential areas, and originally referred to an individuals home
   or apartments that were surrounded by a "wall" and can be deemed to be
   "closed off" from the surrounding public domains. The rabbis of the
   Talmud developed a means to render a larger area as a private domain
   by surrounding it. Such an enclosure is called an "Eruv", more
   specifically "Eruv Chatzayrot" or Sheetufe M'vo'ot. The Hebrew word
   "eruv" means to mix or join together; an Eruv Chatzayrot (henceforth
   just "Eruv") serves to integrate a number of private and public
   properties into one larger private domain. Consequently, individuals
   within an Eruv district are then permitted to move objects across the
   pre-Eruv public domain-private domain boundary.
   
   The laws of Shabbat distinguish four domains, which are defined both
   by the manner in which each type is enclosed and the manner in which
   it is used. The first is a makom petor, or exempt area. An exempt area
   is one that is at least three hand-breadths higher than the ground and
   whose area is less than four hand-breadths by four hand-breadths.
   There are no limitation upon transferring an object to or from an
   exempt area on Shabbat. The second type is a semipublic, or "neutral"
   area, neither strictly public nor private, known as karmelit (e.g.,
   fields and oceans). The third type of area is the private domain,
   which in order to qualify must be very clearly set off and defined
   (e.g. the interior of a house). The fourth type of area is the public
   domain, an open area always used by the public. Included in this
   category are highways, deserts, and forests. The Sabbath laws
   regarding the permissibity of transferring objects from one domain to
   another are explained in the Talmudic tractate Shabbat of the Order
   Mo'ed.
   
   Eruvs serve to create a larger private domain. In order to consider an
   area a private domain, the area must cover at minimum an area of about
   12 square feet and must be somehow demarcated from its surroundings,
   either by a wall of some sort or by virtue of its topography (that is,
   it is either all higher or all lower than its surroundings).
   
   The problem was that it is impractical to build a continuous solid
   wall around a community. However, the Rabbis noticed that doors are
   permitted within walls, and that a doorway consists of two parts: the
   vertical members and the lintel on top. In fact, a wall may have quite
   a few doors, and still be considered to enclose an area. In the
   limiting case, there are many doorway openings and having very little
   of solid wall remaining.
   
   This is what happens in an Eruv. The door post function is fulfilled
   by telephone (utility) poles (serving as vertical members), with the
   lintel being cables strung between the poles. However, for a door
   post/lintel combination to be acceptable, the lintel must rest
   directly above the top of the doorposts. Note that this is not the
   typical approach in utility poles, where the cable is attached either
   to the side or to a member held away from the pole. To address this,
   there is often a thin rod attached onto the pole to serve as the door
   post "surrogate" ("lechi"). Additionally, the line that serves as the
   lintel needs to be the lowest of the lines on the pole. If it is not,
   then it is necessary to string a new length of line between the
   affected set of poles.
   
   In areas where the poles and lines do not exist, new pole/line
   combinations must be erected. These added poles must of course be high
   enough so as not to impede traffic. Fences may be used as part of the
   boundary without modification; however, if the ground is eroded
   beneath the fence to any significant degree, the space must be filled
   in. Lastly, all the areas to be enclosed must be "residential areas,"
   or areas suitable for residential areas. It is not permitted to
   include bodies of water [lakes, streams, and ponds, although
   reservoirs currently in use as drinking water sources are permitted
   without modification), and cemeteries. Such areas must be excluded
   from the Eruv by closing them off (either by not including them in the
   Eruv area, or by encircling them within the Eruv).
   
   The Eruv is generally designed by encircling a community with a
   continuous string or wire. There are numerous regulations concerning
   the placement of this wire. Those who live in and use an Eruv have an
   obligation to ensure the Eruv is intact before taking advantage of its
   presence. Usually, there is a group that maintains the Eruv that
   provide such information, and conducts weekly inspections.
   
   Note: There are other types of Eruv than that described above.
   Specifically, there is an Eruv Techumim, which may be used to define
   the "home" location for Shabbat in order to alter the permitted travel
   area.
   
   Note that Eruvim are typically found in traditional communities.
   Eruvim are less of concern to Conservative Jews, and are not of
   significance in Reform. However, the non-Orthodox groups generally do
   not protest Eruv (although some secular Jews do), as it enhances
   Shabbat for those that do observe the laws concerning carrying.