The Common Core State Standards is an effort to create a single set of standards to guide curriculum and assessment of U.S. students in grades K-12. Individuals states have the option to support these standards.
The first sets of Common Core standards cover math and (English) language arts. Here are the standards.
As of late 2015, 42 states along with the District of Columbia, the US Virgin Islands and the Northern Mariana Islands had agreed to adopt the Common Core standards for mathematics and English language arts. (States not adopting these Common Core standards are: Alaska, Minnesota, Nebraska, Oklahoma, South Carolina, Texas, Virginia).
Two independent consortia are developing assessments for the Common Core. These are: the Partnership for the Assessment of Readiness for College and Careers (PARCC) and the SMARTER Balanced Assessment Consortium (SMARTER Balanced).
As of November 2011, 26 states had joined together to develop Common Core standards in science. This program is called the Next Generation Science Standards (NGSS). It is working with the National Research Council, the National Science Teachers Association, the American Association for the Advancement of Science and a facilitating group called Achieve.
Here's a cool project that tries to translate the English language Common Core standards into specific lessons and activities: the Common Core Curriculum Map project. The first edition map is free; you have to subscribe (around $20/year, less for teachers) for updates.