Bonded means that the lead core is chemically bonded to the copper jacket, helping the bullet hold together better.

However, it also means the bullet doesn't expand as quickly. I've shot truckloads of deer with shockwaves; only one was a bonded bullet. All the non-bonded bullets performed well but that bonded bullet did NOT expand. I posted some pics on here last year. Now that deer was DRT but IMHO the bonded bullets are for elk, not deer.