DLL stands for dynamic link libraries and they are important files for some or
all of your windows applications. They are generally software routines to
handle functions that are not originally part of Windows and allow it and the
programs you run on it to do new things. Unfortunately, some of these files may
belong to programs that you have deleted so you may never know which ones are
in use and which ones are relics of old software. Still, better to be safe than
sorry.