Online reputation is becoming more and more important - it is more and more the first perception people have about a company, brand, business, and increasingly people.
Online reputation, like in person reputation builds up over time. It depends upon your company, business, brand or person's digital "portfolio", footprint or profile. It is less and less based on what you, or your company are saying about yourself, and more about what other people say about you. It's what appears in the search results when a person searches for you and what people are tweeting and posting about you.
The greatest online reputation threats for companies are negative media coverage, negative reviews, feedback, customer complaints including posts on grievance sites and negative tweets and posts from dissatisfied customer.
While it is true that a disgruntled employee can post negative reviews, posting from one person usually does not result into that post showing up high in search results - it is usually an accumulation of negative feedback from a large number of people which results in a post or a review getting enough referral or buzz score for it to show up on first page of search results.
after the wikileaks scandal, leaks of confidential information is also perceived as a large threat to online reputation.
