Need details about APIs for customization of Tridion Content Delivery

Am just 6 months old to Tridion. I know the basic stuff related to this tool. I read somewhere saying we can enhance the functionalities of Content Delivery by extending it using some Java APIs.

Could someone guide me where can we get these Java API's from. It would be great if you can quote some examples for this.

What is content delivery API?

The Content Delivery API (CDA), available at cdn.contentful.com, is a read-only API for delivering content from Contentful to apps, websites and other media. Content is delivered as JSON data, and images, videos and other media as files. The API is available via a globally distributed content delivery network (CDN).

Is Tridion headless?

Tridion supports headless content delivery through a system component called Dynamic Experience Delivery (DXD).

Apart from the official documentation (http://sdllivecontent.sdl.com/LiveContent/content/en-US/SDL_Tridion_2011_SPONE/concept_35CC2F9DCEA34995931E181C165FCC0F, login required) there is an excellent series of articles on the topic by Mihai Cadariu: http://www.sdltridionworld.com/articles/sdltridion2011/tutorials/extending-content-delivery-storage-sdltridion-2011-1.aspx.

I suggest you start from there and let us know if you have more specific questions!