In dummies terms, affiliate marketing is a business you can start that allows you to sell any type of product from another business that will pay you a commission or percentage of any sales that you can generate.
You can earn anything from a few dollars to thousands of dollars per sale but this would obviously depend on what program or service you promote as an affiliate.
The best way to start would be to try and find products or services that are related to your current hobby as you will already know about this subject and you will know what products and services people need within this subject.
In today's world you cannot just simply promote your affiliate links as searchers know this business and will not click your links.
You would either need a domain name that would forward to your affiliate links or a website that provides free information on your subject with affiliate banners on your site.
People actively search for information and want this information for free so if you provide free information or informative articles on your subject a reader will become loyal to it and eventually will trust you enough to purchase products or services through your links.
You must build a relationship with people involved and try establish yourself as an expert within your niche.
There are many ways you can promote your website or domain such as yahoo answers, social bookmarking websites, social networks, forums and article directories.
Use these services to establish yourself within the subject of your choice.
The more you know about a certain subject, the more you will be able to provide people with valuable information and in turn will increase your sales.