Gmail is the free email service from Google, which is integrated into its suite of web apps. It's available to anyone who signs up for a Google account.
Web-based email accounts were once looked at as second class, but with the arrival of Gmail that began to change, and today it's probably the the email account to have.
Unlike other web email apps, Gmail has a very minimal and ad-free homepage. When you sign in, you immediately see your inbox, instead of having to click through to it, as with Hotmail. The left bar shows your email folders and contacts. If you choose to be signed in to Google Chat, your online contacts will also appear here, and you can chat to them without leaving Gmail.
Emailing is, of course, fast and reliable, with excellent spam filters to keep your inbox clean. The settings in Gmail are second to none; you can turn on keyboard shortcuts, allow images to be shown automatically or not, set a signature and even an out-of-office auto reply. There are lots of themes too, so if minimal isn't your thing you can brighten Gmail up a lot!
Offline allows you to access Gmail even if you can't get online, while Yahoo!, Hotmail and other mail services can be imported and integrated into your Gmail inbox. There's also a great area called Labs, which is full of new experimental features that you can try out.
The only thing Gmail doesn't do is drag and drop, which you can use to file your mails in some other services. However, it is still the ultimate email web app. When you add in the integration with the other excellent Google web apps, you have an incredible service. Gmail offers user-friendly and customizable email, which makes the service streets ahead of the competition.