As Google describes it in their announcement:

Gmail Labs is a way for us to take lots of the ideas we wouldn't normally pick and let you all (who use Gmail) decide whether they're good or not. When you sign in, you'll see a new page in Settings called Labs. It has a list of experimental new features, and you can enable or disable each one. Some of the popular ones will become core parts of the product, and we'll eventually retire the ones that don't get much use. We've put feedback links in there, too, so you can discuss a feature with other users and the engineer(s) who wrote it.

They give users an escape hatch just in case something's broken; always a nice thing to have.

There are currently 13 "features" in the Lab list.

Quick Links

Adds a box to the left column that gives you 1-click access to any bookmarkable URL in Gmail. You can use it for saving frequent searches, important individual messages, and more.

Superstars

Adds additional star icons. After enabling this feature, you can choose which icons you wish to use in the "General" Settings page.

Pictures in chat

See your friends' profile pictures when you chat with them.

Fixed width font

Adds an option to the reply dropdown menu that lets you view a message in fixed width font.

Custom keyboard shortcuts

Lets you customize keyboard shortcut mappings. Adds a new Settings tab from which you can remap keys to various actions.

Mouse gestures

Use your mouse to navigate with gestures. Hold right-click and move the mouse left to go to a previous conversation, move it right to go to the next conversation, and move up to go back to the inbox view. Works best on Windows.

Signature tweaks

Places your signature before the quoted text in a reply, and removes the "--" line that appears before signatures. Can't use this and the "Random signature" Labs feature at the same time.

Random signature

Rotates among random quotations for your email signature. Can't use this and the "Signature tweaks" Labs feature at the same time.

Custom date formats

Adds options to the general settings page allowing the date and time format to be changed independent of language. For example, you can use a 24-hour clock (14:57) or show dates with the day first (31/12/07).

Muzzle

Conserves screen real estate by hiding your friends' status messages.

Old Snakey

Kick it old school with Old Snakey! Enable keyboard shortcuts and hit '&' from the main page to play a game of snake.

Email Addict

Lets you take a break from email and chat by blocking the screen for fifteen minutes and making you invisible in chat.

Hide Unread Counts

Hides the unread counts for inbox, labels, etc.

Some of them are cute, but in terms of usefulness, they aren't all that exciting. Still, play around with them and let both Google and me know what you think.

Source: By Tech Ex http://technologyexpert.blogspot.com/