Just as the 68030 had been an evolution of the existing 68020, the 68040 was an evolution of the 68030. Rather than re-invent, Motorola added more features to the existing architecture and, for the first time, incorporated a floating point unit (Note: The 68LC040 had the floating point unit removed to cut costs).
Like the 68030 and 68020, the 68040 was a 32 bit processor but, apart from the FPU, brought very little new to the party. Seperate memory management units were added for instructions and data, and the instruction cache was increased from 256 bytes to 4kb. It may not have sounded like much of a change on paper but in reality 68040 equipped Macs would be the most powerful 68K Macs ever released.
The 68040 also spelled the end of the road for the 68K Macs and despite Motorola going on to produce the 68060, Apple entered into a joint venture with IBM, the result being a whole new generation of machines based on a little chip called the PowerPC...