Umbraco stores identifiers in UDI format for most Umbraco object types. This identifier stores all of the metadata required to retrieve an Umbraco object and is parse-able within text. Example: umb://document/4fed18d8c5e34d5e88cfff3a5b457bf2. UDI's can be used in many of the querying APIs.
UmbracoHelper is the unified way to work with published content/media on your website. Whether you are using MVC or WebForms you will be able to use UmbracoHelper to query/traverse Umbraco published data.