UPDATE (27JUN2017): I've been ping'd a few times about updating this post since it uses the old portal UI. It seems many people land here through their favorite search engine. John Gallant has an updated post here that should server the same purpose: http://blog.jongallant.com/2017/03/azure-active-directory-access-tokens-postman/
Postman (https://www.getpostman.com/) is a popular tool for testing out a web api. Figuring out how to use it with a resource protected by Azure AD is a bit daunting for many. I created this walkthrough video to help you understand how to use the postman oauth 2 authorization helper with AAD.
Here are the values I use in the video so you can copy/paste them:
Auth URL: https://login.windows.net/common/oauth2/authorize?resource=https%3A%2F%2Fb2byour.resource.domain%2F
Access Token URL: https://login.microsoftonline.com/common/oauth2/token