A virtual assistant is someone who provides professional support services to clients from a home office.  Typically these services include administrative tasks such as making appointments or replying to emails, technical tasks such as building a website or managing a social media account, research-related tasks such as conducting market research or real estate research, or personal tasks such as writing thank you cards or making dental appointments.
Part of what makes this job so appealing is that it can be done from home. Most of the work is either conducted online or over the telephone, making it possible to work from anywhere as long as you have access to a telephone and a reliable Internet connection.
If you want to become a virtual assistant, there are a couple of different routes that you can take. First, you can try contacting companies or business professionals directly to see if they need your services. This tends to be the hardest route to find clients, simply for the fact that you have to hard sell your services. However, it can also be one of the most lucrative options since it allows you to set your own rate.
Another option is to search for clients online through one of the many freelancing websites. The only downside to going this route is that there is a lot of competition for jobs on these sites, which can drive the prices down. That means you may have to drop your prices significantly if you want to compete with other workers.
If you have any personal connections in the business world, that can also be a good place to start. Talking to people you know about whether or not you can help them be more productive is a great way to find new clients. 
Finally, you can consider working for a company that connects virtual assistants with clients. These companies handle the hard work of finding clients for you. Typically the clients submit jobs to a pool of virtual assistants, allowing you to pick and choose the tasks that you want to complete. The only real drawback to this option is that the wages are usually far lower than you could earn on your own since the parent company takes a portion of the fees. However, for many, the convenience factor of not having to hunt for clients helps offset the lower pay.
Not sure if you have the right skills to become a virtual assistant? Check out this list of some of the many tasks that VAs perform to see if there is anything that is a good match for your talents: