Answer:
Judicial activism and judicial restraint are opposite approaches to legal and constitutional interpretation used as the basis for decision-making in a court case. The terms are usually, but not always, applied to US Supreme Court decisions and to the ideological leaning of certain justices or Courts as a group.
Judicial Activism implies the Court majority applied personal or political ideologies that go beyond the intent of the legislation or the constitutional amendment(s) being used to justify the decision. The underlying belief in accusations of "judicial activism" is that the Court overstepped its authority by "legislating from the bench," thereby violating the constitutional separation of powers.
Judicial Restraint implies the Court majority acceded to the wishes of Congress, upholding legislation and adhering to the doctrine of stare decisis (relying on precedents). Judicial restraint is commonly associated with the concept of "maintaining the status quo."
As criticism, both judicial activism and judicial restraint may be based on the political bias of the person applying the label, and may or may not have a foundation in reality. The term, "judicial activism," is usually used as a pejorative, and is often thought to be associated with liberal/progressive Courts or decisions (the Warren Court is usually cited as activist), but conservative Courts are just as likely to be activist as progressive courts.
One recent example of conservative judicial activism is the US Supreme Court's decision in Citizens United v. Federal Election Commission, 555 US ___ (2010). In finding for the petitioner, Citizens United, the Court amended the Bipartisan Campaign Reform Act of 2002 to loosen restrictions on campaign financing and promote the First Amendment rights of corporations. In issuing its decision, the conservative faction of the Roberts Court advanced the cause of "corporate personhood" by allowing large (even multinational) corporations the ability to drown out the voices of the common electorate through the production and distribution of quasi-media messages promoting or denigrating targeted political candidates. In granting corporations this ability, the Court opened the door to making corporations the most influential purveyors of public opinion, without requiring truthfulness or accountability. Since the Republican Party is typically aligned with big business interests, progressives (and many moderate conservatives) consider this decision a dangerous example of conservative judicial activism.
Justice Stevens wrote a thoughtful and comprehensive dissent to the majority opinion, which was joined by Justices Ginsberg, Breyer and Sotomayor.
For more information, see Related Questions, below.