Nero is a burning application. It can burn many "disc" files, include ISO, IMG, BIN/CUE, as well as create new discs from files on the computer (no "disc" file is needed). For DVD-Video, in Nero, you simply load the DVD-Video template, and then drop the VOB/IFO/BUP files in the VIDEO_TS folder.
is a program that extracts "disc images" from DVDs (and will remove copy protection at the same time, if any exists). As an added bonus, recent versions (starting sometimes in 2003-2004) also have the ability to burn the images back to a new disc.
So, both can make DVDs, but they operate differently, and may have different source file requirements to make the disc (DD needs images, Nero can take images or just files).
Guides for burning and copying are found in the AUTHORING/BURNING section of the digitalFAQ.com guides.