Blogs are diaries that are made public via the Internet. They have come into increasing prominence, mostly because some people think that they provide journalistic and socio-political content - which is not necessarily true.
The word 'Blog' was derived by shortening the term 'web-logs', so the original creator (unknown to most) may have intended that they be used as logs rather than diaries, which would suggest that they'd be less personal and more factual in nature. Presently you can come across both kinds in the blogosphere (a loose collection of all the blogs available on the World Wide Web), with some people using their blogs to objectively record observations, links, news, weather and the like, and others using them to record their thoughts and analyse events of the day in a more subjective manner.
Blogs and their contents have existed long before the terms 'blog' and 'blogging' (writing and posting blog entries) were coined; since the inception of the World Wide Web, sites have existed where certain groups of people have noted their observations and thoughts in a regular manner. However, blogging in itself relies on the myriad of blogging software and services available (which include Blogger (a fee service) and TypePad (a paid-for service), while current software includes iBlog). These simplify the act of blogging, since once you set up your service or software properly, you can pretty much type in whatever you want published and rely on the service/software to do the rest. This has enabled most people to concentrate their efforts on content creation rather than content management, thus leading to the proliferation of Blogs.
Tips on Blogging
No one can really teach others how to write a blog entry - people can at most give pointers about how to do it. Most of what follows is for those who are writing subjective blogs; objectivity simplifies things to an extent where writing becomes trivial. Most good blog entries are written when you don't concentrate on writing the blog entry itself and instead just relive the day's events, analysing them, and finding the funny ones to write up. Most people don't have very interesting days, but even normal, mundane life can seem captivating if presented in an interesting way. You could try being humorous about things or you could start giving one of those TV news type analyses about your day, or something similar (there are a billion ways to do it, and using a unique way really helps you out with the writing and thinking part of it).
Implications and Historic Context
Some people also use their blogs to say things or act in ways in which they can't in real life because most readers understand that the rules of engagement are different online. However certain actions can still cause damage and horror stories of damage caused by blogs abound, including a story about how some guy was fired from Microsoft, because his blog entry showed Apple G5s being delivered to Microsoft's headquarters. That is not to say that blogs can only or invariably lead to bad publicity. There are plenty of stories in which people have managed to become published in 'old media' because of their blogs, including Salam Pax, the author of Dear Raed, whose blog entries from Baghdad in the middle of the second Gulf War made him famous worldwide and got him a book deal. The thing to remember is that blogs are pretty much like diaries of old, such as Samuel Pepys' Diary and probably even Anne Frank's diary (though of course that wasn't originally written for public consumption). Much like these diaries, blogs are prone to both praise and controversy. In essence, blogging has simply democratised the process of publishing everything from your thoughts, to your own Pepys-styled diary by simplifying the process of giving a large section of society and the world access to your writing.
People say writing a blog encourages thinking, because it forces you to remember interesting things that happened throughout the day. You should try and collect your thoughts long before you sit down to write your blog, because trying to collect thoughts while writing the entry itself makes you concentrate too much on the task at hand. Blogging is a little like flying; you are at your best when you don't pay any attention to how you're doing it.