Email client
In many cases you don't have just one email client. For example, if you're using Gmail as your email provider, you might use the default Mail app on your iPhone, Airmail on your Macbook at home, and when you check your personal email at work (hypothetically, noone ever does that, of course), you open gmail.com in your web browser.
Generally speaking email clients come in two flavors; the ones that are downloaded to your device or the ones that you access via a web browser.
In the above example, Mail app and Airmail are downloaded applications on your phone and laptop (even if one came preinstalled), while gmail.com is a browser-based application.
Email client market share
Getting a complete overview of the email client market share is hard. Most email clients don't share their usage statistics, and - opposed to the web - it is unreliable and error-prone to install analytics into an email.
However, som providers do attempt anyways and published their findings. The following is a list of sources that provide some insight into the market share of email clients.