Normally when you connect to the Team Foundation Service you are presented with a web page to log in with your Microsoft Account (aka LiveID). When you log in you can choose to have it remember you and you won’t have to enter your Microsoft Account credentials again (unless you don’t log in again for a long time, and then you’ll be prompted again).
That’s great for humans, but what about an application or another web service that wants to connect? For that the code will need to use “alternate credentials,” which you must enable on your account. Then we can write some code to connect to the service with those credentials.