If you were asked this question, how will you reply?
If you have no real reply, this is what gmail is.
Gmail is a free searchable e-mail (web mail) service provider, which is owned by google inc and accessible by the general public using a web browser or the gmail app.
In other words, Gmail is a web service provider which enable it’s user send and received electronic mail (e-mail) via web browser of mobile app.
Though gmail provides it services free to its user, it is still a commercial web service provider because gmail support paid advertisement.