Opportunism generally describes taking advantage of some opportunity without any worry over how it may affect others, especially when there is some great risk of a negative consequence. Usually when people talk about opportunism, they mean it in a very negative sense. Positive displays of opportunism can occur and are often a question of perspective and context.
When people act in an opportunistic way, they are often required to ignore any kind of honor or principle in their behavior. Societies tend to create rules of conduct, but the rules can make it more difficult to achieve things. In fact, some rules are actually created so that everybody has to deal with the same difficulties, which makes for a fair playing field. Sometimes if someone moves outside those rules, they can gain a huge advantage over others who are following them, and this is a prime example of opportunism.
Some people tend to misuse the word opportunism by associating it with any kind of opportunity—just because people are taking advantage of an opportunity, it doesn't necessarily mean they're doing anything overtly selfish, and that is not the context that opportunism was generally meant to be used in. For example, when a person calls in and applies for a great job, he is taking advantage of an opportunity, but it doesn’t become opportunism unless the act itself has some obvious potential to negatively affect another person. An example of opportunism that might contrast with the previous example would be if a person spread negative information about another person to steal his job. Even if the negative information is true, spreading it would usually be considered opportunistic, especially if the information was gained in a private context.
Politics is one of the main areas where the concept of opportunism comes up. In many cases, politicians face situations where they can exploit some kind of opportunity to hurt their political opponents and increase their power. Sometimes these are legitimate advantages, and sometimes they can be ethically questionable. In some contexts, politicians are actually expected to be opportunists, especially by their most fervent supporters, and they can even be criticized for being unwilling to take advantage of chances to hurt their opponents. Politicians who avoid being opportunistic can be at a significant disadvantage against those who indulge, and some argue that being too careful in this area is actually unethical because it slows down progress on important political agendas.