Chapter 2 of William Stallings’ book gives a detailed account of computer evolution and performance, giving examples of each generation and some statistical data.
Second generation computers evolved with the replacement of the vacuum tube by the transistor. A transistor is smaller, cheaper and does not let off as much heat as a vacuum tube. It is a solid state device made out of silicon.
The introduction of transistors allowed for more complex arithmetic and logic units and control units, as, with their smaller size, more transistors could be used in these components.
With these more complex components allowing for handling of more instructions, high-level programming was used and system software was provided with the computer.