Gmail is arguably the core of Google Apps. Most users find that they spend the majority of their time using Gmail. Gmail was created by Google engineers who were frustrated at the poor email clients they were forced to use. It was designed with productivity, simplicity and flexibility as its core ideals. 

There are some significant differences between Gmail and other email clients. Once users understand the purpose and role of these differences, Gmail becomes one of their favorite tools. 
Note: clicking on a link will take you to the corresponding section of the Google Apps Training Center.
Aps CT Study Tips: 
  1. Understand key differences from Outlook.
  2. Master connecting accounts
  3. Understand personalization options.
    • Signature
    • Vacation responder
    • Themes
    • Inbox views
  4. Understand the difference between global and local groups. 
Major differences in Gmail: 

1. Archive vs. Delete
Most older email clients have a limited amount of space allocated to each user. Not so with Gmail. Space is virtually unlimited therefore there is no need to delete email messages. Instead, use the "archive" button to remove messages from your inbox without permanently deleting them. 

2. Search vs. Sort
Older email clients allow you to sort messages by clicking on header rows: date, subject, size, etc. This is a very inefficient way to find email. In Gmail, there are no columns to sort messages. Instead of sorting, you search for messages. 

3. Labels vs. Folders
Most users have extensive folder structures to keep their files and emails organized. These folder systems are built the same way that physical filing systems are created-- one document can only occupy one folder. In Gmail, tags take the place of folders. The good news is that a single email can have multiple tags applied to it.
4. Conversation View
Most email users receive a tremendous volume of email. In an effort to reduce the clutter of the inbox, email replies are grouped into "conversations" instead of being listed separately.