Reach is a generic term for the distance from which a character can make a melee attack. It can also refer to a weapon property that gives increased reach. The basic reach is 1, meaning the attack can only target adjacent creatures. Monsters and some player characters have a natural reach beyond 1. The right weapon or power can extend one's melee reach as well.
Increasing your reach does not necessarily increase your threat range for various effects. For example, you still can make opportunity attacks only against enemies adjacent to you and can flank only enemies adjacent to you, unless another effect says otherwise. Some feats such as Polearm Flanker can address this issue to some degree.
However large a creature's reach may be, a reach melee attack is still a melee attack, not a ranged attack. Thus reach attacks do not provoke opportunity attacks any more than do normal melee attacks.
Weapons with the Reach property have a melee reach of 2 instead of 1. Therefore you can make melee attacks against enemies that are 2 squares away from you as well as against adjacent enemies. As stated above, this alone does not increase the range at which you may make opportunity attacks (see threatening reach below) or flank. However, the added reach can be very tactically valuable, making reach weapons frequent favorites.
Other Increases to ReachEdit
A player's weapon is not the only way he can increase his reach. For example, some class attack powers offer increased reach for their attack, such as the Assassin's Inescapable Blade power. The Polearm Master paragon path offers +1 reach after spending an action point, while the Stoneblessed path gives the character a permanent bonus to reach (even to unarmed attacks). The Eternal Defender epic destiny can also increase reach permanently.
Such bonuses typically stack with each other, potentially creating a character that can make melee attacks approaching some thrown weapon ranges.
Some particularly nasty monsters have threatening reach. This ability says that the monster can make opportunity attacks against any enemy within its reach (not just adjacent targets). This can make it difficult to simply get within melee range of a monster because it can take opportunity attacks against you as you approach.