Until the mid 2000’s CD pressing plants and record labels requested CD masters to be delivered on high quality CDR media. The final replicated CDs that the plant produces are exact duplicates of this master CDR. But now, because internet speeds as faster, the more common way is to deliver the songs individually as WAV files or as a DDP image file.
The difference is that when you send your masters as individual WAV files, any custom song to song spacing that the mastering engineer created are lost and you must use a default 1 or 2 second pause between songs. This is usually fine, but sometimes the artist needs some songs to be laid out back to back or needs a long pause between two specific songs, for example. In this case, the pressing plant needs a DDP file that duplicates the mastering engineer’s layout.
Also a DDP file embeds CD metadata such as artist name, album title and song names. This may not be a problem with some CD pressing plants that let you enter that data online, so you can still send them individual songs as WAV files, but sometimes they require a DDP file to embed CD metadata.
If you only plan to release your album digitally, say on iTunes or Bandcamp, you do not need a DDP file, only your individual song masters as WAV files.
Check with your CD pressing plant or record label to see if you need a DDP file. About half of my clients request one. If you decide not to get a DDP file, you are usually fine, but if you decide later that you need one, I can produce that for you at a later date. I save all client projects for at least several months.