Difference between Portal, Portlet, Blog, Forum ?

A Portal is a web site that acts as hub, gateway or window to other information and services on the Web. Common Web portals are Yahoo, Netscape, etc.

A Portlet is a specialised content area that occupies a small window of a portal page. Portlets from different sources (Portlet Providers) can be integrated into the portal framework. Example portlets: weather info, news flashes, stock tickers, etc.

 A Blog is Yours, and A Forum is Not

Blog, you set up your account and site, design it to look the way you want it to look, add advertising in some cases, and write regularly. Over time, both by your own design and as a result of what you've written, your blog develops a "personality". You can elect to allow comments or not. Blogs are often devoted to a specific topic, although some are devoted to a number of topics (for example, "All the things I've thought" or "What I did today") that, essentially, are only related by virtue of the fact that the same person wrote them.
A forum belongs to someone else (a company, an online site, a non-profit organization, or anyone who who creates and runs the forum).

A Forum is a collection of posts offered by the general public, often in a question-and-answer format although some forums are nothing but comments on a post that, itself, was nothing but a comment.