Archives are files that contains other files and folders
. Most of time they are compressed, which means that a file in a archive takes less space than the same file outside of the archive.
However, archives can become cumbersome if you use it to store your everyday documents. They can be useful in some cases to store documents or send many files at once in a email instead of attaching ten different files.
7zip is an excellent software that is able to open almost any archive format.