Is it possible to sum up Alfred North Whitehead's metaphysics in a sentence? No. But here's a sentence anyway:
There are no concrete generalities.
For philosophical materialism, matter is a concrete generality. It is, quite simply, what is. In Whitehead's thought, there are only concrete particularities. That is, 'actual occasions.' But isn't actual occasion just then another way of saying 'matter'? I think not. To say that reality is constituted by actual occasions is to say absolutely nothing about what those occasions are like. Neither hot nor cold, wet nor dry, material nor immaterial. That's really rather the point of them. Contrast this to 'matter,' which is said to have all kinds of properties. For some it is dead, lifeless, inert. For others, it is active, lively, vibrant. These are variations on the same theme: matter as a concrete generality. On the contrary, actual occasions demand specificity – it is built into their very definition that nothing much can be said about them in general. Matter, then, appears as an abstraction. And there's nothing wrong with abstractions. As Whitehead affirms, one cannot so much as think without abstractions. However, we have to be careful as these things can get out of hand fast. 'Actual occasion' can then be understood as the minimal concept that permits wide-ranging abstract thought while safeguarding against the poison of concrete generality – i.e. against something fundamental, general and capable of being meaningfully described.
I'm going to be reading a lot more Whitehead in the near future, so we shall see whether this understanding is still standing in a little while. At the moment, this is basically an abridged version of my understanding of Isabelle Stengers' understanding of Whitehead.