Sorry for the delay in my reply :). Okay, I'll try and break this down a little bit more:
A freelancer can be in any field, it simply means that they are hired for specific projects or tasks and are not employees of those who hire them.
Online freelance opportunities often include writing, transcription, data entry, answering services, research, translation, graphic design, online marketing, etc.
Skills depend on the type of freelancer you are, and may range from phone etiquette, organizational capabilities, an understanding of SEO, the ability to execute efficient searches and determine what is relevant information, proficiency in more than one language, writing skills in an in-demand language, design skills, etc.
Check out some freelancing marketplaces such as Elance.com, Guru.com, or oDesk.com to see some of the types of jobs listed there, that should give you a more complete picture of what people are currently hiring for.
As for investing money -- some of the sites have subscription fees, though I know Elance has a free option that offers enough flexibility to get started and earning before any money has to go out. Unless they've changed it, oDesk was even more open on that. Guru lets you open an account for free and familiarize yourself with the site, but you can't really do anything for submitting proposals without a subscription. You might be able to find some worthwhile free freelancing sites with a search, these are just the ones I'm familiar with, though make sure that they all offer some sort of Escrow service to make sure you get paid for the jobs you do. Hope this helps :).