A new profile in Facebook is a blank-slate. You have the opportunity to fill in as many blanks as you like, make friends, join groups, add applications, play games, and become fans of companies or organizations.
For more details on the differences between Pages and Groups, check out this article: Facebook Pages vs. Facebook Groups
Every change you make to your profile is reversible-- you can easily replace a bad profile picture with a better one, change your hobbies, leave a group, or get rid of applications when you get tired of them.
Facebook touts itself as a "social utility that connects you to the people around you"-- the service itself is based on friendship and networking. Finding friends and making new acquaintances in Facebook is easy. Through the Find Friends service, you have options to import names from your email contacts, search by school/network/workplace, or browse by name. When you request friendship of someone in Facebook that person has the opportunity to review your request-- he or she can accept, deny or ignore your friendship. New friends can recommend additional friends to each other, add names to lists (if you want to be able to distinguish between friends and family, for example), or send messages to each other. New friends can also indicate how it is that they know one another.
Applications are the extra tools, games and doodads that you can add to your Facebook account to increase or decrease your productivity. Applications can help you bring information from other applications into Facebook (Flickr pictures, blog posts, YouTube videos) or they can bring new ways of communicating with your friends to your profile. Facebook applications can be created by anyone for any purpose, and Facebook makes suggestions, lists new applications and shows the most popular selections by Facebook users.
See a group you're interested in joining? If the group is open to everyone, feel free to click Join this Group. Your group memberships can be published to your Wall where your friends can see them and you can also receive notification when others join groups, add new content to a group page or respond your posts on a group page.
When you become a fan of a Facebook Page, you can post items to the page, make comments and more. When you become a fan of someone or something, news of this action appears on your Wall and in the NewsFeed where your friends can view them.