SAML 1.1/SAML 2.0 is the default token format when using ACS as the authentication service for your website. In this model, your website talks to ACS using WS-Federation protocol and what it normally gets back is a Saml token. This scenarios is fairly straight-forward as WIF natively supports WS-Federation protocol & SAML1.1/SAML 2.0 token formats.
There are cases where you might want to return a Simple Web Tokens (SWT) after a successful authentication. For example, you might want to use this same SWT (available as a bootstrap token) to call other downstream REST/OData services as depicted in the following diagram.
ACS fully supports returning an SWT token after a successfully WS-Fed authentication but WIF currently doesn’t support SWT tokens. You would have to write a custom Security Token Handler for WIF to process SWT tokens coming back to your website. I have created some extensions which enables this and other OAuth WRAP related scenarios. Feel free to download the code from my SkyDrive.