It is basically writing your review in black and white about something you like it or hate it or share your user experience. It can vary as widely as from your Mobile, Cars, Airlines, Hotels, Books, Music Players, laptops etc.
However in the current Web 2.0 world, we need a search engine to search for Opinions. For instance, you can find tons of user Opinions about Reliance Broadband on this blog. Their are more than a thousand people writing Product Reviews. So in the context of Web 2.0, we need a specialized engine to read Opinions.