If you're building a web app that integrates with Dropbox, you should send the user through the OAuth app authorization flow, where they can choose whether or not to authorize your app to access their account. You can find information in the OAuth Guide, as well as the authorization documentation.
When you configure the /oauth2/authorize URL, where you send the user to Dropbox, you can set the 'redirect_uri' to tell Dropbox how to redirect the user back your app after they authorize it. Check out the documentation for more information.
