Gmail is Google's free web-based email service that allows users to send and receive emails. To use Gmail, users must first create an account which involves choosing a username and password on Google's website. Gmail offers features such as spam filtering, searching, built-in chat, storage of over 7641 GB of space, and security through HTTPS encryption. Users can access Gmail from any browser on desktop or mobile devices and use additional features like video calling and customization of themes.