Buuieza is the result of Super Buu absorbing Frieza.
When Super Buu made a hole to another Dimionsion when he was in the Hyperbolic Time Chamber he ended up in Hell. While in Hell Super Buu encountered Frieza who wanted to fight him, Super Buu easily beat Frieza and then absorbed him to gain more power. It turned out that Buuieza was weaker than Majin Buu.
Buuieza went to Earth to exterminate the human race which he was but before he could Buuieza got in a fight with Super Saiyan Gotenks. At first Buuieza was getting the upperhand in the fight with Super Saiyan Gotenks but after Gotenks used his Super Ghost Kamikazie attack Buuieza was about to die but he used Death Ball on Gotenks the attack almost hit Gotenks but he was able to dodge it. Buuieza soon died after he used his attack.