The second generation of computers lasted from 1959-1964. It was a very short era. The vacuum tubes were replaced by transistors. Actually, the invention of a transistor had a huge influence on the development of electronics.
The invention of transistor allowed computer to became smaller, faster and cheaper. These computer also were more efficient and more reliable that the computers of the first generation. The first symbolic (assembly) programming languages appeared in this generation. Also, the backgrounds of high level programing languages were developed. The first versions of COBOL and FORTRAN appeared.
The computer instructions storage also changed. The step from magnetic drum to magnetic core technology was made.