You may have seen this chart on the actual, perceived, and ideal distributions of wealth in the US.
Wealth inequality refers to this unequal distribution of financial assets among US residents. It’s related to income inequality, but refers to what you keep as opposed to what you get. Income inequality has also been increasing in the US. If you’re wondering where the numbers come from or why this is a problem, check out this video:
Washington Post: How fighting income inequality became Obama’s driving force
Wikipedia: Economic Inequality
Thompson and Smeeding, Inequality in the Great Recession – The Case of the United States (PDF)
BBC: The Wealth Gap – Inequality in Numbers (UK)
Feel free to include more links in the comments.