Storing certain types of data often results in unnecessarily large file sizes. For example, a plain-text file contains thousands of pages of text could produce a file that would take minutes to transfer across the internet. Compressing such a file would likely see the file transferred in seconds.
There are many different types of compression algorithms; some are generic to all file types (like .ZIP) whereas some are more effective with particular file types (audio for example).
A compressed file can contain multiple files and folder inside. The files can often be of any data type. Once a file is compressed, it will need to be decompressed before the original data can be accessed.