The release engineer's duties are many and varied indeed. The usual definition I've heard is that release engineers are responsible for everything between development, QA and the door (e.g. actually putting a CD into the customer's hand or making the software available for download.).

Actual duties vary wildly from company to company. In many cases release engineers simply burn CD after CD and manage creaky old Makefiles and/or build scripts.

In some cases though release engineers do quite a bit of software development themselves, although often in a scripting language like Perl or Python or even Java depending on the software the company produces and other technical/environmental issues.