Windows 10 Email (built in)
Microsoft's built-in program for email in Windows 10 is called, simply, Mail:
Mail is really an "integrator" that allows you to consolidate email from more than one source, which can be a great convenience if you need to juggle more than one email account. It provides all the basic functions of email: receive, send, store drafts, create folders, signature, basic font formatting, and basic spell-check. It also has a sync capability to synchronize with other email accounts (Yahoo, Gmail, Verizon, etc.).
Other Email Programs
An email client is a software program that you would use to view, save, and organize your email. If you are using Yahoo mail, Gmail, or any of several other similar programs, you are using a web-based client, and you can continue to use them and ignore Mail completely if you wish. There are also email clients which can be installed directly on your computer (Microsoft's Outlook is one of them). Windows 10 will accommodate these as well.