It couldn't be easier to get started: pick your gender, type in your name and e-mail address, and presto! You've got a one-person family tree. From there, you just click on easy-to-follow icons to add parents, siblings, spouses, and children, with as much detail as you care to provide. The best part? You can unload some of the work on other members of your tree. Provide an e-mail address for any family member, and Geni will allow them to log in and contribute their own knowledge--from adding new family members to filling in minute biographical details.
It's definitely not a tool for "serious" genealogists, but for those of us who just want to tinker with a quick, simple, and fun family free, it makes for at least a few diverting hours.
Note: If you need some help tracking down distant or deceased relatives, or you have a more serious interest in genealogy, the following resources are a great place to start:
Genealogy at the National Archives