Is pdflatex an engine or a format?

In comments I listed all the "latex" formats that got installed by default in my texlive 2012 installation.

texmf-var/web2c/eptex/platex.fmt 
texmf-var/web2c/euptex/uplatex.fmt 
texmf-var/web2c/luatex/dvilualatex.fmt 
texmf-var/web2c/luatex/lualatex.fmt 
texmf-var/web2c/pdftex/cslatex.fmt 
texmf-var/web2c/pdftex/latex.fmt 
texmf-var/web2c/pdftex/mllatex.fmt 
texmf-var/web2c/pdftex/pdfcslatex.fmt 
texmf-var/web2c/pdftex/pdflatex.fmt 
texmf-var/web2c/xetex/xelatex.fmt 

One has to be careful though as they are not all "latex" in the sense of being a compiled version of an unmodified latex set of macros.

I believe latex.fmt and pdflatex.fmt only differ in that the same latex.ltx file was dumped with tex and pdftex respectively. (Actually latex.fmt is these days dumped with pdftex in dvi etex mode rather than classic tex) But for example cslatex is a different format, made by compiling the tex macros in

 /usr/local/texlive/2012/texmf-dist/tex/cslatex/base/cslatex.ini

This file makes some small number of definitions setting a non standard encoding regime if I recall correctly, and then inputs the standard latex. This is all above board and as expected, you can do just about anything you want with the latex sources except call a changed copy latex (and if you read the small print you can even do that if you claim you must) pdfcslatex.fmt is of course the same source as this, but dumped with pdftex. I'd need to check but I think the xelatex and lulatex formats similarly make some small (or not so small) additions to the standard latex macros as well as being dumped by the xetex and luatex engines respectively.

pdflatex is a terminal command that tells the pdfTeX engine to use the LaTeX format.

Correction

Strictly speaking, pdflatex loads the pdfLaTeX format, but as far as the user is concerned this is identical to the LaTeX format. The additional commands that are available when using pdflatex (mostly starting with \pdf) are provided by the pdfTeX engine, not the format.