Gmail is Google's version of email. Users send/receive email, keep a contact list, and keep track of Tasks (to-do lists). Email is easily searchable and can be filtered to find messages quickly using the Search bar at the top. Users can also create labels and archive emails to organize the messages.
By switching the "Mail" header in the upper left-hand corner to "Tasks," users can create to-do lists and set reminders for themselves. Users can also directly add email messages to Tasks by selecting the email and clicking MORE at the top.
Hangouts is available in Gmail. Users can call, videochat, and/or instant message other users by initiating a message in the lower left-hand corner of the screen.
There are many custom features available for Gmail users that can be accessed in Gmail's Settings by clicking the gear-icon in the upper right-hand corner.
Google Help Center: Gmail