A Reprint is when a card which has been featured in a previous pack is re-released in a different pack or as a limited edition in a special edition box or a collective pack such as Dark Beginning 1 or for the OCG Beginner's Edition 1. An example card is "Exodia the Forbidden One" which first appeared in Legend of Blue Eyes White Dragon but was reprinted in the Dark Beginning 1 Booster Pack.
Usually the rarity of a card remains equal or declines when featured in a Reprint-Pack. For example, all pieces of "Exodia" were first released as Ultra Rares in Legend of Blue Eyes White Dragon, but only the "head" remained an Ultra Rare whilst the other pieces are Common in Dark Beginning 1. In some cases the rarity of cards increases, such as with "Fusion Gate" and "Coffin Seller". Increasing the rarity of a card has become somewhat common recently due to the releases of Special Edition cards and Turbo Packs.
Reprints often serves to bring back a card a player usually need (Staple, Toolbox or Splashable cards). The most notable case is "Mystical Space Typhoon" with more than 20 reprints, albeit most of them are as Common.
Reprint also serves to errata the card: Either to update the text to the latest version (Problem-Solving Card Text) or modify its effect. The latter category serves to lower the power of a cards on the Forbidden List, and thus, making them more usable. Most notable cards include "Future Fusion" and "Ring of Destruction".