In professional wrestling, championships may be unified to consolidate the number of championships in a given promotion, or to add legitimacy and prestige to a certain title's lineage. In a title-for-title match, one of four things will happen:
Both championships will be remain active and be defended and lost together under one name, but title changes will reflect each individual title's history (WWE's Unified Tag Team Championship is a good example of this).
The J-Crown, a combination of several lightweight championships from various wrestling promotions (including the WWF Light Heavyweight Championship), was defended mostly in Japan and Mexico. The title has since been abandoned and all belts returned to their home promotions.
The WCW Cruiserweight Championship was briefly unified with the WWF Light Heavyweight Championship on July 30, 2001 when Light Heavyweight Champion X-Pac defeated Cruiserweight Champion Billy Kidman. The titles continued to be defended separately and X-Pac lost the Light Heavyweight Championship to Tajiri a week later.
Triple H holding the WCW and WWE Championships as the WWF Undisputed Champion
The WCW Championship and the WWF Championship were unified to create the WWF (later WWE) Undisputed Championship at Vengeance 2001 when Chris Jericho defeated WWF Champion Stone Cold Steve Austin and WCW Champion The Rock in the same night. Essentially, the WWF Championship became the Undisputed Championship while the WCW Championship was retired, although the belts used to represent the two championships would adorn the Undisputed Champion for several months afterwards, up until a single belt was introduced by Triple H in 2002. After the brand extension, which split the WWF roster into two brands (Raw and SmackDown), the Undisputed title served as the prime championship for both groups. The title again became "disputed" when champion Brock Lesnar announced he would only defend the belt on SmackDown. The following week, Raw General Manager Eric Bischoff announced the creation of the World Heavyweight Championship. The WWE Undisputed Championship immediately became the WWE Championship.
The WWE European Championship was unified with the WWE Intercontinental Championship in July 2002, when Intercontinental Champion Rob Van Dam defeated European Champion Jeff Hardy on Raw in a title unification match. The European title was abandoned as a separate championship.
The WWE Hardcore Championship was unified with the WWE Intercontinental Championship in August 2002, when Intercontinental Champion Rob Van Dam pinned Hardcore Champion Tommy Dreamer. The Hardcore Championship was abandoned soon after.