A. It is an enclosure that in the view of Jewish law transforms an area into a private domain, making it permissible to carry things within that area on Shabbat. There are complicated laws as to how to make an eruv; the bottom line is that unless you know the eruv is there you would probably not notice it.
Its background philosophy has been explained in these terms:
“Man has created for himself a society whereby he leaves the ‘closeness’ of his family and home, and moves out into a world, a social world. Now, on Shabbat one may not do creative work, therefore it may be said that one may not reactivate any part of a creation (e.g. turning on a light = reactivating physical energy). So, on Shabbat, since man has created a society, he should not go out into the world reactivating his society. Since the basic function of a society is trading, therefore if a man carries anything from his private domain into a public area, he reactivates the basis of society because by moving anything into society, he reactivates the potentiality of becoming concerned in the society of his weekdays.
“The eruv surrounds an area, whereby it encloses a person from the outside world, so that he may carry. Within that area it is as if the ‘weekday’ society no longer functions and since the world is ‘refused entry’ into the area, carrying would reactivate nothing and is hence permitted.”