The processing of a set of data in order to reduce its size. Compression may be performed both to reduce the amount of storage space occupied (say, to fit the data onto a single CD) and to reduce the time it takes to transmit (say, over a slow telephone line). Compressed data must be decompressed by reversing the process before it can be read or modified.
When you compress computerized information, you make it smaller (taking up less space on the disk), meaning that less data is needed to represent exactly the same information. Using a compression utility, you can compress files stored on disk so that they take up less disk space and leave more space for other files. Some programs have the ability to compress data that's being held in memory, allowing the computer to keep more data in memory and thus spend less time retrieving data from the disk. And some modems and communications software can compress the data they send back and forth to one another. Since there's less data, it takes less time to transfer them.
Keep in mind that even when you are archiving files you never want to compress your original and only file. All it takes is to lose one bit, one electronic signal, from a compressed file and that file is destroyed. Bits get lost all the time. In an uncompressed file, it's a minor problem, but in a compressed file, it can be a catastrophe of considerable dimension.
There are many known compression/ decompression ALGORITHMaSn,d the search for better ones has become commercially important since most new communications technologies (such as digital television) can only work with effective data compression.
All compression algorithms work by uncovering and eliminating redundancy in the data, and there is an important distinction between those that preserve all the information in the data (loss/ess methods) and those that sacrifice some information for greater compression (lossy methods). Lossy algorithms such as JPEG and MPEG are suitable only for final delivery of data to end users, as the information losses accumulate each time the material is recompressed and decompressed.