Simply put, it is about the newer generation of websites that are about "User-generated" content. Examples are YouTube, Digg, Flickr, Wikipedia etc. If you notice in each of these websites, the owners of the website have simply provided a platform for us, the users to add content from our side. The website, with the help of all the users, thus becomes a huge database of information!
Blog is short for weblog. A weblog is a journal (or newsletter) that is frequently updated and intended for general public consumption. Blogs generally represent the personality of the author or the Web site. May or may not be interactive.
Blog Talk Blogger: any contributor to a blog Blogging: the processes of creating or maintaining a blog multi-Blog: an online "conversation" taking place between several blogs
Blogs are set-up like conventional Web sites, with navigation links, and other standard Web site features. Blogs have one standard characteristic, however: the posting. Blog postings are text entries, similar to a diary or journal, which include a posting date and may include comments by people other than the author, photos, links, or other digital media.
Twitter is for staying in touch and keeping up with friends no matter where you are or what you’re doing. For some friends you might want instant mobile updates—for others, you can just check the web. Invite your friends to Twitter and decide how connected you want you to be.
How does Twitter work?
When you send in a mobile text (SMS), Twitter sends it out to your group of friends and posts it to your Twitter page. Your friends might not have phone alerts turned on so they may check your web page instead. Likewise, you receive your friends mobile updates on your phone.
Wiki - website that allows the visitors themselves to easily add, remove, and otherwise edit and change available content, and typically without the need for registration. This ease of interaction and operation makes a wiki an effective tool for mass collaborative authoring.