From Diablo Wiki
Respec is slang for respecialisation and refers to a game feature that allows players to partially or completely reallocate the points they have spent in various skills, 'talents', 'attributes', 'feats' or other character values. Respeccing in Diablo 3 will allow players to reallocate most or all of their skill points, thus allowing high level characters to change their play style.
This was not a feature offered in Diablo 1 or 2, where characters could never change their stat or skill points once they were placed. (Respecs were added to Diablo 2 in the v1.13 patch.)
Respecs are a player-friendly feature found in most RPGs, notablyMMORPG, .
 Diablo II Respecs
From Patch 1.13, Diablo 2 players can now respec; there are two ways to perform a respecialization.
The first such way is to complete the Den of Evil quest, granting one respec per difficulty completed in 1.13. If this quest is completed before 1.13, one free respec will be given to players on Hell difficulty.
The second way to get a respec is to obtain the four essences as random drops from the Act Bosses.
- Twisted Essence of Suffering from Andariel or Duriel
- Charged Essence of Hatred from Mephisto
- Burning Essence of Terror from Diablo
- Festering Essenece of Destruction from Baal
Combining this four objects in a Horadric Cube will create a Token of Absolution, granting the desired respec.
 Diablo III Respecs
Read more aboutrespecs in the main DiabloWiki: