A blog is basically an online diary. People and companies use blogs in many ways – some using them to record events that have happened in recent days, while others use it more like a news / events calendar or discussion board.
If you have a website then you should consider adding a blog to it. Often when you have a website built you find that you do not need to do many changes to it for the first 3 or 6 months at least, and while this may seem great to you, it does not always look good in the eyes of the search engines. Search engines such as Yahoo, Google and Bing look for fresh relevant content on a website and use that as part of their ranking algorithm. You may not need to change the content on the main pages but by adding a blog or news section, this allows you to add relevant content to the site on a daily or weekly basis.