The Facebook Timeline was introduced about a year ago, in Spetember 2011. It was a major change in the Facebook design of the personal profile. At first it was not mandatory to switch from your old profile look to the new timeline, but over time Facebook automatically changed the profile look on all accounts. By now, everybody on Facebook uses the new timeline, even pages.
The main purpose of the timeline is to give the users a fresh and structured view over everything that other users post on their “wall”. All activities, like when you add a friend or change your status, will be shown chronologically on your timeline, so others will have an easy overview over what is happening to you.
It can be a hard decision, what to put on the cover photo. But whatever your plans are, keep in mind that this should be considered as a cover of the book you have just written. It in a way represents your life story so choose your timeline cover photo wisely. It will reflect who you are to those who will visit your profile. You can only make one first impression, so make it good. Put on something that you really love, like your family or your hobbies.
This is a great way to customize the looks of your profile as much as you can. Not like with Myspace, you can not modify anything else besides your cover photo and profile image.