Let’s take a look at MetaMask.io’s description of themselves:
"MetaMask is a bridge that allows you to visit the distributed web of tomorrow in your browser today. It allows you to run Ethereum dApps right in your browser without running a full Ethereum node.
MetaMask includes a secure identity vault, providing a user interface to manage your identities on different sites and sign blockchain transactions.
Our mission is to make Ethereum as easy to use for as many people as possible."