"Filesharing" is when you share files between computers over the internet.
There are many different ways to share or transfer files from one location to another including using Peer-to-peer software (Bittorrent, Emule, Limewire), File Transfer Protocol, WebDav and many others.
Any files obtained via filesharing must be treated as suspect and virus checked before opening.
Not all software available by filesharing is copyright infringed*; in fact the free Linux operating system is often distributed using Bittorrent.
However, filesharing to obtain copyright infringed software is now the most common way people's system are infected by viruses.
AP do not support copyright infringement in any way.
*Piracy is a colloquial (and legally incorrect) term for copyright infringment.