eMule is a software developed since 2002, available only for Windows, that allows to search and download content from the Internet. It uses peer to peer technology and this improves availability of files and speed of transfer.
Basically, when an user transfer a file, that file is downloaded from multiple computers that are sharing that file and at the final all parts are put together automatically by eMule and the desired file is ready to use.
Is an equivalent of LimeWire and offers the option to search files by name or category. Supports multiple downloads in the same time.