Long ago, some packages used to make the '@' sign active, so that special measures were needed to type it. While those packages are still in principle available, few people use them, and those that do use them have ready access to rather good documentation.
Ordinary people (such as the author of this FAQ) need only type '@'.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=atsign