There are a wide variety of e-mail programs, known as e-mail clients.
Typical ones include Outlook, tlook Express, Thunderbird and Eudora.
Outlook and Outlook Express (part of Windows) are Microsoft products: www.microsoft.com
Thunderbird is part of Mozilla, www.mozilla.org
Apart from e-mail programs, e-mail is commonly accessed using a Web browser in what is called webmail, such as Internet Explorer.
A popular webmail program is Gmail available in Google, www.google.com