In old-school 40k, Bonebreakas were a type of Ork heavy tank. Used by all Ork Clans, a Bonebreaka was armed with a single heavy Battle Cannon in a turret and a huge spiked Deathrolla on its front hull. The spiked Deathrolla is what gave the Bonebreaka its name.
The Bonebreaka was similar to the Bonecruncha in that they both shared the same chassis which typically consisted of a looted Imperial vehicle such as a Rhino. They only differed in terms of main armament. Whereas the Bonecruncha consisted of two twin-linked double-barrelled weapons meant for rapid fire and destroying infantry and lighter vehicles, the Bonebreaka was armed only with a massive Battlecannon meant for annihilating enemy armor and tanks.
The Bonebreaka (and the Bonecruncha) were considered to be true Ork Heavy tanks. Nowadays this concept has been mothballed by Games Workshop, presumably because it has been superseded by the new Battlewagon kit and its variants.
Then in 5th Edition...
A brand new Battlewagon kit was finally released in 2009, coinciding with the focus on Orks as the main "enemy" faction in 5th edition. As of 2020 in 8th/9th Edition, Bonebreakas are considered to be a sub-type of the Battlewagon dubbed as Krushas. This family of wagons also includes similiar and nostalgic assault vehicles such as the Bonecrusha, the Spleenrippa and the Gitmasher. The Bonecrusha and Gitmasher are vehicles that have never been seen or mentioned previously, so they may or may not be a different configuration of the Bonebreaka or a different vehicle entirely. In any event, the current incarnation of the Bonebreaka combines armored transport capabilities with an arsenal of heavy firepower and an enormous spiked deffrolla to create a versatile and enduringly popular Ork assault vehicle, albeit one that comes at a somewhat high points cost.
A lot of extra gubbinz and power are required to keep the deff rolla turning. This can make Bonebreakas somewhat prone to breaking down, but most crews are savvy enough to take preventative measures, perhaps dragging a dedicated grot rigger on-board to perform repairs mid-battle. Others reassure themselves by painting their vehicle with blue glyphs, hanging lucky squig’s feet in the driver’s cabin, or feeding captive grots and snots into the tank’s workings so as to keep their Bonebreaka’s belligerent beast-spirit satisfied.