The goal of the Common Core State Standards is to define what all students need to know and do in order to be college and career-ready when they graduate. According to the official web site, they "include rigorous content and application of knowledge through high-order skills," they are "informed by other top performing countries," and they are "evidence-based."
Common Core v. current Pennsylvania standards
3rd grade math
Current standard: "Solve problems using addition, subtraction and multiplication (straight computation and word problems)."
Common Core: "Use multiplication and division within 100 to solve word problems in situations involving equal groups, arrays, and measurement quantities, e.g., by using drawings and equations with a symbol for the unknown number to represent the problem.
7th grade English
Current standard: "Make inferences, draw conclusions, and make generalizations based on text."
Common Core: "Cite several pieces of textual evidence to support analysis of what the text says explicitly as well as inferences drawn from text."
Source: www.corestandards.org, Pennsylvania Department of Education