Webmail is email that is accessed through a browser, via a webmail client - which is a web-based program for reading, sending and managing emails.
There are two types of webmail client.
Webmail service providers allow you to create email addresses based on their domain name ( .....@gmail.com or ....@yahoo.co.uk or ...@outlook.com).
They also can (depending on the service) allow you to manage your domain-based email through their service.
The second type of webmail clients are webmail applications.
These only work with domain-based emails. The most common examples of these are Horde, Roundcube and SquirrelMail. You will see them if you use the webmail services associated with your cPanel.
Although it is perfectly possible to access your domain-based email accounts through one of these webmail applications, it is more common to configure a desktop client or a webmail service provider to access your domain-based emails.
The reasons for doing that are:
- The facilities of webmail service providers are much better than webmail applications.
- Spam control is better.
- Webmail service providers integrate with calendars and other applications.
- You can manage multiple email accounts through a single webmail service provider account, where you need a separate login for each email address if you use a webmail application.
(There does not seem to be standard terminology for these various types of program, but I think the terms used here - webmail client, webmail service provider and webmail application - make reasonably intuitive sense.)