What is DDS?

DirectDraw Surface, which is how game textures are stored for the graphics card. Block-compressed so the GPU can read it without unpacking, and usually carrying a whole chain of mipmaps. Open a game's files and this is most of what you'll find.

What is PPM?

Uncompressed RGB behind a four-line text header - about as simple as an image format can be while still being one. Nobody ships PPM to users; it exists as the thing scripts and teaching code write when they'd rather not link an image library.