Freelancers are the people that offer different types of services and get paid "per project". In the world wide web there are millions of websites that need content, and they need more content than what they can produce.

Since they need content and cannot produce it in a timely manner, they look for other sources (this method is known as "outsourcing"). This is where freelancers come into the scene.

Freelancers offer their skills in exchange of a determinate fee, and get paid after they deliver the finished work. By repeating this process, earnings can be very good and consistent.

Normally, freelancing is taken as "writing for someone else" (ghostwriting), but actually it covers a variety of fields that can fit your skills, such as translations from/to other languages, website building (strategic placement of content) advice, marketing advice, Website creation/design, graphics creation/design, software development, and a lot more (you can even get paid to post in forums).

The possibilities are so many, that everyone who owns a computer and knows how to use it can become a freelancer.

Freelancing doesn't necessarily requires an investment, but paid advertising can normally get you better job offers.
But there are some good free options to promote your services, such as marketing forums and freelancing Website that do not require you to pay a fee to become a member.

Another good thing about freelancing is that you are your own boss. You can accept or reject jobs depending on how much they are willing to pay or simply how much you like the job.

Despite taking orders from other people, you are still your own boss. You work whenever you want, and you only have to worry about the quality of your work, unless your client sets a deadline for you to meet.

Of course, freelancing is a job, and as such, it has some disadvantages:

Since you earn on a per-project basis, you earn as much as you do, and sometimes quantity and quality cannot be a part of your job together. You have to make quality your priority and then look for quantity, otherwise the word of the poor quality will be spread and you will not get more and better jobs.

Depending on the job, you will have to work for hours to finish a project. Even if it's "free"lancing, and it gives you a lot of flexibility, it's still the real life, and if you do not work, you do not get paid.

Expanding your business it's also very hard, so it all comes down to earn as much as you work.
To get more orders, you need to market your services in an efficient manner.

Also, you may hear about better earning possibilities with less work using the same marketing methods, the decision is yours, but freelancing is great for short-term earnings while you explore other options, and even you can invest the money you make out of freelancing on them.