Neighbors are players that can speak to each other. Unlike Masons, there is no guarantee made of any Neighbor's alignment. It is up to the moderator's discretion whether they can only speak at night, or at any time.
In addition, Neighbors have been known to possess other power roles, or even have powers that belong to the group as a whole.
See also Neighborizer.
Neighbors, regardless of when they are allowed to speak to each other, are considered Normal on mafiascum.net. Under the current Normal guidelines, there are no restrictions on combining power roles, and Neighbours are no exception to this; in fact, Neighbours were exceptionally allowed to have additional power roles even under former rulesets in which combining power roles was disallowed.
Use and Power
Neighbors are essentially unconfirmed Masons where the moderator honestly acknowledges no expectation of one Neighbor having the same alignment as any other Neighbor. In an ideal world, the alignment of any Neighbor would be as random as the alignment of the player itself. In practice - and especially in Themed games - Neighbor is seen as an excuse to mix scum Neighbors where they would have been called bastard for doing so with a Masonry. The name for this excuse was originally coined by PokerFace
To that end, the real power of Neighbors comes from their ability to speak to each other. Scum players, particularly those that do not like playing as scum, will not be able to maintain a pro-Town appearance for very long if engaged in dialogue like if they were a Mason. Thus, in the hands of skilled players a Neighborhood can function either as a Masonry (if both players are town) or a directed Cop investigation (if one is not town). In the hands of unskilled players, Neighbors are virtually useless.
Thus, Neighbor is a fairly low-tier power role that amplifies the skill of its players. This does not mean that you can stuff a game with them - if the Neighbors deduce that they are all town, they will begin to act like Masons. Masons, to contrast, are quite powerful.
If you are one of three Neighbors, chances are reasonable that one of the other Neighbors is scum.
If you are one of four Neighbors, chances are very good that at least one of the other Neighbors is scum.
If there are multiple Neighborhoods in the same game, chances are very good that at least one of them has a scum Neighbor.
If you are one of more than four Neighbors, it is nearly certain that at least one representative of every anti-town faction is in the Neighborhood.