"Common Core is a set of education standards -- for English and math -- that were adopted in 2010. Forty-five states and the District of Columbia decided to follow the new guidelines. Alaska, Nebraska, Texas and Virginia opted out, while Minnesota decided to only adopt the English standards (with others perhaps joining them soon). These states began implementing the standards last year.
The standards haven't created national curricula -- state and local educators are still in charge of that. Instead, they establish benchmarks that make it easier to compare how students are doing across state lines, and to make sure students across the country are learning the same essentials."