Socialism Is Central Planning & the Coerced Redistribution of Wealth
There is no single definition of socialism, and various policies and programs can be socialistic in nature without the entire country being socialist. Furthermore, socialist countries fall along a spectrum from weak socialism (1970s Britain) to strong socialism (the Soviet Union). It should be noted that Marx makes no distinction between socialism and communism; indeed, Marx's "scientific socialism" is communism.
By and large socialism is central planning on a grand scale; e.g. our public education system. To be clear, I don't advocate doing away with our public education system, but I recognize that it is socialist in nature, a monopoly mandated by the federal government and every state government. Predictably, without competition the quality of public education has decreased significantly over the past several decades. Vouchers anyone?
The Wikipedia article Nick smugly suggested I use to brush up on the "actual" definition of socialism correctly points out that socialism is a system of government in which property is largely owned or controlled by the state rather than by individual citizens. A closer reading reveals that socialism also involves wealth redistribution. Presumably, Nick and others will recall Obama's desire to do just that. Indeed, a reading of Obama's two autobiographies reveals that, during college, he attended, and was sympathetic to, several socialist gatherings and policy positions. Furthermore, there is nothing in Obama's actions and policies that suggest his favorable view of socialism has changed. For example, he still favors socialized medicine.
Nick claims that Warren Buffett has been called a socialist. I'm not aware of that or the reasoning behind it, but it seems like a red herring to me.