Its short for Web-log and in its simplest form its an online diary that you share with others, for example if you went to Corfu for a week and wrote about what you did each day and then published this to the net, that would be a Blog. Similarly you could do a weekly one on any subject you fancy, its very open-ended, just about the only rules are that you publish on line and there are many sites such as Blogspot which give you all the tools you need for free.