JoinMarket, just like Bitcoin, doesn't have a single point of failure. It can be understood as a protocol that facilitates certain kinds of collaborative transactions—CoinJoins—via an open market.
Creating collaborative transactions is not a technical problem, but a problem of coordination. Someone has to bring the participants together, coordinate time and place, as well as amounts and fees.
Usually, this problem is solved by a central coordinator. In the best case, the central coordinator is non-custodial and blinded, meaning that user funds are never at risk, and the amount of information the central coordinator can obtain is minimal. However, even in this optimal case, the central coordinator is still a single point of failure.
In addition to being a central point of failure, any central coordinator usually charges a fee for the coordination service, which means that it is an entity that is making money. In most cases, this entity is a company that can be pressured and subverted, as has happened in the past. Even without any outside pressure, an economic entity might make certain decisions against the best interest of its users, just out of self-preservation alone.1
JoinMarket provides a decentralised alternative to these central coordination services. It is not a central entity, it is software that is coordinating the actions of multiple peers, just like Bitcoin. JoinMarket is neutral, meaning that the system itself is not charging its users to use the software for coordination. Fees are earned by market makers, i.e. those who are willing to provide liquidity to the open market. Consequently, JoinMarket is not a financial entity. Only the users are making money. JoinMarket is not.
JoinMarket is a tried and tested CoinJoin implementation, having been actively used on Bitcoin's mainnet since 2015.2 Because of its lack of a centralised coordinator, it works a bit differently than other CoinJoin implementations.3 You can read about the details of JoinMarket's design in the high-level design document of the JoinMarket docs.
JoinMarket is free and open-source software, which means that anyone is free to use, inspect, and modify the software.
Because of its decentralised nature, and thus the lack of funds and focused effort that a central company structure allows, the JoinMarket software was, historically, a bit difficult to set up and use. We hope that the efforts around Jam will make it a bit easier.
See, for example: https://archive.ph/C53tk, https://archive.ph/hIYJO ↩
See the Bitcoin Wiki for https://en.bitcoin.it/wiki/JoinMarket ↩
Shinobi, 2021 JoinMarket Vs. ZeroLink ↩