A computer in the modern sense is a technological device that can store digital information, transform the information (via programming) and output the information in a number of different ways ( on screen/printing/movement).
Computers rely on a wide range of electronic components to do all these functions.
On a basic level a computer consists of a central processing unit (CPU), memory (volatile vs non-volatile like flash memory), input devices and output devices.
Computers today are integral to to everyday functioning of the modern world. In particular millions of computers are connected to each other to produce the Internet.
Computers come in a variety of scales or sizes for various purposes from handheld computers usually called smart phones, or tablets, laptops, desktops to larger scale supercomputers. Traditionally though the term computer generally meant a desktop or a laptop scale device.