The Pirate Bay is a search tool for torrent files and magnet links. It became hugely popular several years ago, as a way of finding torrents and magnet links that are up to date and could be downloaded conveniently in a torrent download tool.

Thepiratebay.org (TPB) is one of the largest and best visited torrent sites, millions of visitors per day. It's commonly used to download pirated content.

It depends on how you use it and where. It's also blocked in certain countries. So as long as you download copyright-free content and you're using TPB in a country where it's legal, you're all good.

The site is, however, still active.