TCDL is a technology-neutral language you can use in your templating code. Depending on the Output Format of your Component Template, TCDL tags get transformed into your Web site's language at publish time or at request time.

TCDL is a technology-neutral language for SDL Tridion dynamic functionality.

When a user publishes content that contains any kind of dynamic functionality, such as Dynamic Link Resolving or Dynamic Content Retrieval, SDL Tridion includes this functionality in the form of TCDL tags (XML elements that have a tcdl: prefix).