Multi-sig wallets are especially popular among cryptocurrency startups and other groups because they require more than one person to agree before any currency gets moved around. It's a safeguard against rogue employees who might want to run off with the money. For this reason, it's also a popular way of storing cryptocurrency raised in initial coin offerings, a new fundraising technique in which investors trade cryptocurrencies like ether and bitcoin for new ones created by companies. Exactly how much cryptocurrency has been locked up because of the bug is unclear, but some cryptocurrency blogs have reported that Parity wallets make up 20% of the entire Ethereum network, which powers the ether cryptocurrency.

http://www.businessinsider.com/ethereum-parity-wallet-hack-freeze-missing-code-2017-11