Google Labs are new features that haven't been officially released. Some are just for fun, while others can be really useful. Keep in mind that these features are still in development - as Google says, "they may change, break, or disappear."
To see Labs, go to Settings (in the upper-right corner of Google Apps), then click the Labs tab. There are labs for email and labs for calendar, so you have to be in Email to see the Email Labs or in Calendar to see the Calendar Labs. Then to activate a specific lab, click Enable then Save Changes. After you enable your first lab, a green beaker will be added to the top of your page next to the Settings link - then you can quickly access Labs by clicking on the beaker.
Adds a box in the left column for 1-click access to frequent searches, important individual messages, and more.
Gives you a variety of stars instead of just yellow stars.
Create an email template for common messages. You can also use this to create multiple signatures.
|Default Text Styling
Change the default font for emails you compose.
|Send & Archive
Adds a button to messages so you can send a reply and archive the message in a single step.
|Google Calendar Gadget
Adds a box on your Gmail page showing your upcoming Google Calendar events.
Want to see the whole year at once? This lab adds a "year view" button to the calendar.
|Free or Busy
Adds a box next to your calendar showing if your co-workers are free or busy right now.
Draws your attention to what's coming up next on your calendar.