An email client is an email program that can be installed on your Desktop and which lets you exchange e-mails. Examples of email clients are Microsoft Outlook Express and Netscape Messenger.
Microsoft Outlook Express is installed as part of your Windows operating system. Netscape Messenger will be available to you if you use the Netscape Browser.
The advantage of using an email client like Microsoft Outlook Express or Netscape Messenger is that you do not need to stay connected to the Internet while you are reading your messages, writing new mails or replying your mail.