A weblog or blog is an online publication with regular posts, presented in reverse chronological order. A blog typically consists of the following components:

• Title - main title of the post
• Body - main content of the post
• Trackback - links back from other sites
• Comments - comments added by readers
• Category - category the post is labeled with (optional, multiple
• categories possible)
• Permalink - the URL of the full, individual article
• Post Date - date: time the post was published

A blog has certain attributes that distinguish it from a standard web page, including these:

• It allows for easy creation of new pages: new data is entered into a simple form (usually with the title, the category, and the body of the article) and then submitted. Automated templates take care of adding the article to the home page, creating the new full article page (Permalink), and adding the article to the appropriate date- or category-based archive.
• It allows for easy filtering of content for various presentations - by date, category, author, or other attributes.
• It (usually) allows the administrator to invite and add other authors, whose permissions and access are easily managed.

Blogs are different from forums or newsgroups. Only the author or authoring group can create new subjects for discussion on a blog. A network of blogs can function like a forum in that every entity in the blog network can create subjects of their choosing for others to discuss. Such networks require interlinking to function, so a group blog with multiple people holding posting rights is now becoming more common. Even where others post to a blog, the blog owner will initiate and frame discussion.

Taken from the Wikipedia, read more.