A blog is the shortened name for Web Log and means just that... a log of events. There are many free blog platforms but the better known are Wordpress and Blogger. Blogs have evolved over the years from being a virtual diary to fully blown websites with sales pages and product offers.
The idea of a successful blog is to get as many regular readers as possible. It's then possible to sell some of these readers a few products that closely relate to the subject of the blog. It's a particularly good and easy way to make money from affiliate offers. It's also a good way to keep in touch with people and keep them up to date with outcomes of events such as competitions. However, Facebook seems to have taken over this role in recent years.
Many online businesses have links to blogs on their websites and therefore they need someone to regularly update them. This creates opportunities for writers who have a few spare hours each week to supplement their income.