What is a blog online?

[J]

originally a blog was somewhat like a place for people to share ideas and thoughts and events with others---then it moved more into the ecommerce area---there are many of the original type blogs still around, but today blogs are typically part of a function of a full website where you could conduct a business, an informational only site, or a lead generation site, or an affiliate site, or an adsense advertising site -- where the blog part is used to keep a 2 way conversation going with your visitors

there are probably almost as many possibilities as there are people