An Informed player is a player, typically Town-aligned, who is given additional information about the setup at the start of the game (in the Role PM). Most commonly, this is the identity of another Town-aligned player. The role is normally written before the alignment, as though it were a modifier, e.g. "Informed Townie".
The Mafia have no need to be informed of players' alignments, but can often benefit from being informed of specific power roles that exist in the game (most commonly to prevent them accidentally doing something that would be good play in most setups, but would expose them as scum in this particular setup). In this case, the information is typically given as a factional ability (as giving it to a single player would be functionally equivalent).
An Unlyncher is similar in effect to an Informed player, as both try to prevent the lynch of a specific player (the Unlyncher because they'll lose, the Informed player because they know it will be a mislynch).
A Friendly Neighbour effectively causes other players to become Informed.
Some bastard games have been known to give information that looks useful but is actually entirely useless (e.g. via giving a method of recognising a role that isn't actually in the game). This can be considered effectively equivalent to a False role. Even when the information is useful, information in bastard games tends to be considerably more off-the-wall than merely giving the identity of a townie.
Use and Power
An Informed Townie is normally slightly stronger than an Innocent Child, for much the same reason a Friendly Neighbour is; the player who's known to be town is effectively immune to lynching (even if the rest of the town believed the information target were scum, the Informed Townie's reaction would likely be "don't lynch them, lynch me instead"!). It's typically weaker than a One-Shot Cop, due to not being able to choose the target (although it gets the information a day earlier). It's also considerably less swingy than the One-Shot Cop would be, because there's only one possibility for the information given (the identity of a townie) rather than three (the identity of a townie, the identity of scum, or no information because the target died overnight or the role was interfered with).