pdfTeX is an extension of TeX which is capable of directly generating PDF output.
Use this tag if your question is about pdf(La)TeX specific problems, i.e. if the subject has something to do with pdfTeX but not with standard (La)TeX which produces DVI output.
For standard LaTeX choose an appropriate specific Tag. Since the majority of questions on this site deal with LaTeX, there's no need for a LaTeX tag. However, if your question is really about LaTeX in general, you may choose the latex-project tag.