I have long found this a little confusing. So often it appears that email services like Gmail, email clients like Inbox and even devices are mashed together to discuss engagment rates. In the new 2017 Email Client Market Share it claims the top three clients are iPhone, Gmail and iPad. iPhone and iPad are devices not clients. Gmail is a service to my understanding.

Can anyone shed some light on why all these differnt things are all called clients? Or shed any light on how to look at these things and understand how to refer to them. Anyone? Thanks in advance.