Twitter APIの OAuthの説明ページ

Authenticating Requests with OAuth | dev.twitter.com

To use the Twitter API, the first thing you have to do is register a client application. Each client application you register will be provisioned a consumer key and secret. This key and secret scheme is similar to the public and private keys used in protocols such as ssh for those who are familiar. This key and secret will be used, in conjunction with an OAuth library in your programming language of choice, to sign every request you make to the API. It is through this signing process that we trust that the traffic that identifies itself as you is, in fact, you.

結局本家が一番分かりやすい