See How Chips Are Made for details on how transistors are fabricated on silicon. The date is the year that the processor was first introduced. Many processors are re-introduced at higher clock speeds for many years after the original release date. Transistors is the number of transistors on the chip. You can see that the number of transistors on a single chip has risen steadily over the years.
Microns is the width, in microns, of the smallest wire on the chip.
For comparison, a human hair is 100 microns thick. As the feature size on the chip goes down, the number of transistors rises. Clock speed is the maximum rate that the chip can be clocked. Clock speed will make more sense in the next section. In many cases the external data bus is the same width as the ALU, but not always.