The default setting for a Facebook profile requires you to approve anyone who wants to be your friend. Until you approve them, they can only see your name, your profile picture, your list of friends and the content on your wall. (Unless you have friends in common, in which case, they will be able to see all of your photos, birthday, and religious and political views.) Once you approve them, Facebook allows you to see their entire profile page, and the same "Friends of Friends" details for their network.
You can change the default privacy settings at any time by going to "Customize settings" under "Privacy Settings" on the Account page.
For instance, Facebook allows you to limit who sees the information that you display about yourself by checking from a dropdown list that includes: Everyone, Friends of Friends, Friends Only, or Customize.
The information for which you can limit access to is:
Things I Share: Posts by me, Family, Relationships, Interested in and looking for, Bio and favorite quotations, Website, Religious and political views, Birthday
Things others share: Photos and videos I'm tagged in, Can comment on posts, Friends can post on my Wall (check to enable), Can see Wall posts by friends
Contact information: Mobile phone, Other phone, Address, IM screen name, e-mail address
Ads display on Facebook pages and some can be tailored. For instance, if you've "liked" a page or event (called a "social action") and an ad for that business or event appears on a friend’s Facebook page, that friend may see your name in the ad. To adjust whether your friends can see ads tailored to your social actions, go to "Account Settings" under Account and click the "Facebook Ads" tab at the top.
If you have enabled any applications like birthdays, notes, or photos, you can adjust their settings by going to "Application Settings" under Account.
You can also limit who can search for you on Facebook for your Profile Picture, your Friend list, a link to add you as a Friend, a link to send you a message, and a list of the pages of which you are a Fan.
You may also block undesirables so they can’t find you in a search, see your profile, or interact with you through Facebook channels. Furthermore, you can block any persons associated with a particular e-mail address.
You may find that games and other applications will become distractions. If your friends post updates about them, you can hide the postings (look for "Hide" in the upper right corner of each posting). The best advice is to not accept the invitations, which you will inevitably receive, to join or try a long list of fun sounding time-wasters.
Facebook Privacy Settings | Facebook Privacy Help