The Big Question: What is the Supreme Court, and how is Barack Obama changing it?

Why are we asking this now?

Barack Obama has nominated Sonia Sotomayor, a New York appeals court judge, to be the first Hispanic – and only the third woman – to sit on the US Supreme Court. The story of her personal journey, the daughter of Puerto Rican immigrants who grew up in the Bronx housing projects but went to Princeton and Yale universities, elicited tears and applause at a press conference to announce her nomination this week. The US press have dubbed it a "rags to robes" story.

Why is there a vacancy?

These are big appointments, because they outlast the president who makes them. Supreme Court Justices are appointed for life. Sotomayor is 54; she could still be on the bench more than two decades from now, when Barack Obama's administration is just the stuff of history books.

She will be replacing David Souter, a liberal judge appointed by the first President George Bush, who announced he wanted to resign just weeks after Obama's inauguration. Justices of a particular political persuasion usually want to hang on until they can be certain they will be replaced by a judge of a similar hue.

What is the role of a Supreme Court Justice?

There are nine of them, including the Chief Justice, who is currently John Roberts, a conservative appointed by George W Bush in 2005. Together, they form the highest court in the land, equivalent to the law lords in the UK, and are charged with interpreting the constitution.

The Supreme Court is the one body that can strike down laws made in Congress, if they are deemed unconstitutional, and it is the ultimate arbiter of complex legal problems that might have numerous interpretations in lower federal courts or in the courts of different US states. The might of the Supreme Court is a powerful check on the legislative (Congress) and executive (White House) branches of government, and in recent years it has extended legal rights to Guantanamo Bay detainees and ruled that the Environmental Protection Agency has the power to regulate carbon emissions, all circumscribing policies of the Bush administration.

Why all the discussion of liberals and conservatives?

In a way that may be anathema to observers from the UK, Americans are very conscious of the politics of their judges, and never more so when it comes to the Supreme Court. The court currently splits 5-4 in favour of those with a conservative interpretation of legal questions, although Anthony Kennedy – a moderate appointed by Ronald Reagan after two previous nominees failed to win support from Congress – sometimes sides with liberals.

Even after the replacement of Justice Souter, only three of its members will have been appointed by Democrat presidents, reflecting the long period of conservative dominance of the White House.

There is a line somewhere between "interpreting" and "making" the law, and politicians – and Supreme Court nominees – are all expected to insist that they disapprove of "activist judges". But you need look no further than the 1973 Supreme Court decision in Roe v Wade, which struck out the country's anti-abortion laws on the grounds that they violated a woman's right to privacy, to see that the Justices' rulings can have profound consequences.

Does that make the Court a major battleground in America's culture wars?

That is certainly how it is viewed by special-interest groups, who put as much money into fighting their way up the judicial branch as they do into getting favourable laws out of legislators. Anti-abortion campaigners hope Roe v Wade could be overturned, or undermined by a conservative Supreme Court; campaigners for gay marriage hope that equal rights could one day be enshrined by the highest court in the land.

Can we expect another big fight over Sotomayor?

Absolutely. Although Sotomayor is not known for any legal philosophy, conservative groups are already going through her rulings as an appeals court judge to see if they can divine any patterns that might galvanise Republican opposition to her appointment and help block her confirmation by the Senate.

Opponents of judicial activism are already pointing to a YouTube video of her remarks, at a law school panel discussion, about the appeals court being "where policy is made". She will also be asked to defend her judgement that struck out claims by a white firefighter that he had been discriminated against because of affirmative action policies in his department. Beyond that, die-hard conservatives are keeping their fingers crossed for other revelations. "I have got my metal detector out," Tony Perkins, head of the Family Research Council, said ominously yesterday. It's what any good conservative activist would do when confronted with a liberal nomination for the Supreme Court.

But she will be confirmed, right?

Barring some unexploded bombs unearthed by the conservative metal detector, that seems the case. Not least of the reasons is that the Republican party is lary of offending the Hispanic community, one of the fastest-growing constituencies in the country and one among which it lost ground in the last presidential election.

What does the President see in Sotomayor?

Mr Obama added "empathy" to the list of characteristics he wanted in his first Supreme Court Justice, alongside the more traditional prerequisites of a wide legal experience and an impressive intellect. That stirred opponents, fretful that empathy with individual appellants might overwhelm a Justice's adherence to the impersonal edifice of American law.

Most of all, though, he sees a life story so compelling that it can steamroller opposition to her appointment. It was hard not to hear echoes of his own "improbable journey" as the president pointed at Tuesday's press conference Sotomayor's mother in the audience, a woman who lost her husband when her daughter was just eight years old and already diagnosed with diabetes, a woman who had worked six days a week to buy encyclopaedias to educate her daughter, and who had seen her win scholarships to the best schools and universities in the land.

What will the appointment mean?

The injection of such emotional appeals into the Supreme Court nomination is a unique and distinctly Obama-esque phenomenon, designed to reach over the heads of the special-interest groups and political hacks of Washington who normally dominate the process. Whether this is a phenomenon that outlasts the current president remains to be seen.

It is not likely that her appointment to replace Justice Souter would change the balance of the court, however. Sotomayor has not judged any significant abortion, gay rights or death penalty cases, nor any on national security, making her likely votes on these cases inscrutable. She is a Catholic replacing an Episcopalian, a woman replacing a man, and the first Latina to ascend the marble steps to the Supreme Court in Washington – but the she is also a liberal replacing a liberal, and this appointment may turn out to be significantly less controversial significant than any Obama eventually gets to make on the departure of a conservative.

Is Sonia Sotomayor's appointment a big breakthrough?

Yes...

* The appointment of the first Hispanic Supreme Court justice is long overdue.

* Sonia Sotomayor's inspirational story takes the nomination process out of the hands of political hacks.

* She could ensure President Obama's imprint on the court for a generation.

No...

* She is replacing a liberal justice, keeping the 5-4 conservative majority the same.

* It is too early to tell how she will vote on "culture wars" issues.

* The main significance of the appointment will be outside the court, cementing Democrat gains among Hispanics.