A program is a sequence of instructions that specifies how to perform a computation. The computation might be something as complex as rendering an html page in a web browser or encoding a video and streaming it across the network. It can also be a symbolic computation, such as searching for and replacing text in a document or (strangely enough) compiling a program.
The details look different in different languages, but a few basic instructions appear in just about every language.
input
Get data from the keyboard, a file, or some other device.
output
Display data on the screen or send data to a file or other device.
math and logic
Perform basic mathematical operations like addition and multiplication and logical operations like and,or, and not.
conditional execution
Check for certain conditions and execute the appropriate sequence of statements.
repetition
Perform some action repeatedly, usually with some variation.
Believe it or not, that’s pretty much all there is to it. Every program you’ve ever used, no matter how complicated, is made up of instructions that look more or less like these. Thus, we can describe programming as the process of breaking a large, complex task into smaller and smaller subtasks until the subtasks are simple enough to be performed with sequences of these basic instructions.