An email client is just any program that lets you read and send email. The most popular client on campus is IMP, but other desktop clients are faster and have more features. For example, you can store messages for offline viewing, and you can manage your address book more efficiently. Desktop email clients include:
If you're configuring an email client, you'll want to use studentmail.williams.edu as the incoming mail server, and mail.williams.edu as the outgoing mail server.