Keynote is Apple's own presentation application. Like PowerPoint, it allows you to put on impressive presentations, but faster and with better end results.
The Adress Book app on your Mac is a great and convenient way of keeping track of phone numbers, email addresses and other information.
Sometimes, even the best of apps get stuck and won't quit, no matter how hard you try. Don't worry, you can always force quit applications.
Guest logins is a convenient and secure way of letting friends (or strangers) use your Mac without letting them access your personal files.