MetaMask's mission, vision and strategy (https://github.com/MetaMask/metamask-extension/blob/develop/MISSION.md):
Mission Making it safe and easy for the most people to use the decentralized web to the greatest degree that is empowering to them.
Vision To realize the highest goals achievable for the human race with the twin powers of peer to peer networks and cryptography. To empower users to hold and use their own keys on these new networks as securely and intelligibly as possible, enabling a new world of peer to peer agreements and economies, in hopes that we may collectively overcome the many great problems that we face together, through the power of strong cooperation.
We provide software for users to manage accounts, for sites to easily propose actions to users, and for users to coherently review actions before approving them. We build on this rapidly evolving set of protocols with the goal of empowering the most people to the greatest degree, and aspire to continuously evolve our offering to pursue that goal. Strategy
Practically speaking, today MetaMask is a browser extension that serves as an ethereum wallet, local cryptographic key store and provider of access to decentralized networks for browser based apps. With more that 1.1 million users, it is the most relied on solution for users and developers of apps built on top of the ethereum blockchain.
MetaMask's visions, and Kyokan-Metamask's goals in contributing to the project, extend beyond ethereum and blockchains. We are excited about the growing movement of decentralized networks, protocols and technologies. MetaMask's potential for extensibility and interoperability can ultimately pave the way for accessibility and empowerment of users across a diverse array of these networks/tech.
The MetaMask extension is a react/redux front end built on top of a javascript core that runs within web pages approved by the user. This javascript core exposes an api to developers that enables their users to cryptographically sign messages and blockchain transactions with their own private keys that are stored securely and locally via MetaMask. Beyond the browser, MetaMask will soon launch a mobile client.
The codebase can found here https://github.com/MetaMask/metamask-extension.