MetaMask

MetaMask is an extension for accessing Ethereum enabled distributed applications, or "Dapps" in your normal Chrome browser.

For Dapp connect to the ethereum network, it needs to connect Ethereum Client or node and this Ethereum node will have access to the ethereum network. Typically , both the dapp and node will run in the same computer.

For instance, take example of standard Ethereum Wallet ( available from ethereum site). Ethereum wallet launches goeth client to connect to ethereum network. Same way, any dapp you write needs a node to connect to the ethereum network.

However, it is very expensive interms of resource usage - node needs a lot of memory and bandwidth. You may argue that you will start when needed, however, it will add up it to the start up time and synching up the before using the app.

To avoid this, you could use the app like metamask where it will turn chrome browser to dapp container. The only different with the above is, the node will be hosted in metamask servers ....

You can also connect to the ethereum network of your choice ( testnet, local network or main network))