About BTDB.in

BTDB is the BitTorrent DHT search engine.

BitTorrent is a file sharing protocol.
DHT is a protocol for searching nodes which distribute a torrent.
Torrent is a file with metadata which describes distributed content.

BTDB's database is formed by DHT users.
BTDB participates in the DHT network and follows the DHT protocol specification.
BTDB is not a tracker and doesn't store any content and only collects torrent metadata (such as file names and file sizes) and a magnet link (torrent identifier).
This means BTDB is the entirely legal system.