Without a program, computer is useless. Program is nothing but a list of instructions which are written by a computer programmer. Execution of such program by CPU causes the computer to behave in a predetermined manner.
The act of writing a program is called programming.
The program’s code is first written in a programming language. Source code of a program is in human-readable form which cannot be understood by a computer. To make the computer understand the code, it is converted into an executable form or binary using compilers, interpreters or assemblers.
The programs can be written in low-level languages or high-level languages. Various high-level languages used to write programs are C, C++, C#, Pascal, Perl, BASIC, Python, FORTRAN, COBOL, LISP, VB, VB.NET, JAVA, ASP.NET, PHP etc.