We can think of Blog as a magazine in real world, In real world magazine has a theme/category/topic, have articles, images and advertisement, and of course readers. A publisher of a magazine strives to give his readers fresh content every month.
In the same way blog can be considered as a shared online journal. A blogger must have an idea about theme/topic of his blog before starting a blog, he must be able to give fresh and useful, unique content in terms of articles images or videos to his visitors. In a blog newest posts are presented at the top and oldest at bottom.
If a blogger succeeds to attract visitors in large he can place ads on his blog relevant to his blog. In this way a blog with good number of visitors can be used as a source of income. A blogger must make sure of integrity of ads he places on his website. A number of services are available on internet which helps bloggers to earn money by assigning them ads relevant to their blog. Adsense by Google is leading example of Ad service available on internet.
You can start a blog with zero investment on Blogspot or Wordpress but with few limitations in appearance and space, but thats more than enough for a new blogger, Later they can buy a domain name,hosting and  freedom in appearance of blog in return of a small investment.
Nowadays from large corporations to small business entities and entrepreneurs everyone is using power of blog to show their presence on internet.There are certain organizations who have their blogs as well as their official websites,because they want to interact with their clients through their blog. Blogging is helpful to establish and maintain a relationship with people in outer world.

After you have launched your blog,its important to make search engine know about your blog's presence on internet.To make our blog visible to search engines and people we do SEO,we use discuss different techniques of SEO to improve visibility of our blog. We will discuss ways of earning through a blog in later articles.