Climate skeptics tend to disagree with the mainstream scientific view on climate change. This can take a variety of forms.
Denying that it’s happening: Some skeptics have questioned whether the planet is really getting warmer and whether humans are really to blame. This view is outside the scientific consensus and is not taken very seriously by most climate scientists.
Disputing the costs: Other skeptics have questioned whether global warming will really be so destructive and whether the cost of warming is actually higher than the cost of reducing emissions.
According to the Intergovernmental Panel on Climate Change, the balance of evidence suggests the negative impacts of global warming will outweigh the positives. And economic models tend to agree that climate change will harm the global economy, especially as temperatures rise. That said, there are plenty of debates over what level of emissions cuts are feasible or cost-effective.
Saying it’s too late: Other skeptics have argued that it’s futile to try to tackle global warming. For example: even if the United States reduces its carbon dioxide emissions, fast-growing countries like China and India will never follow suit. This is a question of politics and not something that scientists can settle.
Wikipedia has a longer rundown of the history of climate skepticism. And the website Skeptical Science has a long list of skeptical arguments (and rebuttals to the arguments that concern climate science).