By default, unlike Windows, Ubuntu comes with a selection of full-powered software programs. This includes an office suite, OpenOffice; a cloud-file syncing service (like Dropbox), Ubuntu One; a photo-manager, Shotwell; Empathy for IM; the Evolution e-mail client, and, of course, a Web-browser, Firefox.
You'll find all these programs, and many more, under the applications menu. From here you can do anything you want with the desktop. Browse the Web, create a document, send an e-mail, talk to a friend over IM, go crazy.
Tip: The one fundamental thing you can't do in Ubuntu's trial mode is make any permanent changes to your desktop, and that includes installing software.
One thing you'll notice, especially if you're running Ubuntu from a CD, is that it will feel slower than your native operating system. That's not because Ubuntu is slow, far from it, it's because when you're running any operating system from an optical drive or a USB stick it's going to be slower than it would be if it were running off your hard drive.
The one fundamental thing you can't do in this trial mode is make any permanent changes to your desktop. Any changes you make during this session will last only so long as the session does. If you turn it off or reboot, you'll be back to the original choice of trying Ubuntu out or installing it. Or, if you've taken the CD or USB stick out, you'll be back to your usual desktop.
That doesn't mean though that you have to lose any work you've done while trying Ubuntu out. You can save files to Ubuntu One, Google Docs or any other online file saving service.
Indeed, many people use live Ubuntu CDs and USB sticks for just this purpose. Advanced users sometimes use USB sticks with 'persistence,' allowing them to make and save changes to the USB stick and effectively carrying their own personalized operating system and files with them in their pocket. So long as they can find a computer to boot up, they can start work with their desktop of choice. For just playing around with Ubuntu though you won't need to worry that.
The other thing you can't do with this trial version of Ubuntu is install software. So, for example, if you want to view an Adobe Flash video, you won't be able to do it because the Adobe Flash player is optional and isn't on Ubuntu by default.
Next page: Using Ubuntu