www.theoldcomputer.com

The the late 1960s and early 70s, there was much talk about "generations" of computer technology. This photo illustrates what were commonly known as the three generations:

First generation: Vacuum tubes (left). Mid 1940s. IBM pioneered the arrangment of vacuum tubes in pluggable modules such as the one shown here on the left. The IBM 650 was a first-generation computer.
Second generation: Transistors (right). 1956. The era of miniaturization begins. Transistors are much smaller than vacuum tubes, draw less power, and generate less heat. Discrete transistors are soldered to circuit boards like the one shown, with intereconnections accomplished by stencil-screened conductive patterns on the reverse side. The IBM 7090 was a second-generation computer.
Third generation: Integrated circuits (foreground), silicon chips contain multiple transistors. 1964. A pioneering example is the ACPX module used in the IBM 360/91, which, by stacking layers of silicon over a ceramic substrate, accommodated over 20 transistors per chip; the chips could be packed together onto a circuit board to achieve unheard-of logic densities. The IBM 360/91 was a hybrid second- and third-generation computer.
Omitted from this taxonomy is the "zeroth" generation computer based on metal gears (such as the IBM 407) or mechanical relays (such as the Mark I), and the post-3rd generation computers based on Very Large Scale Integrated (VLSI) circuits.

http://www.columbia.edu/cu/computinghis ... tions.html