What's the difference between a writ and an appeal?
Writs usually are considered to be extraordinary remedies, meaning they are permitted only when the defendant has no other adequate remedy, such as an appeal. In other words, a defendant may take a writ to contest a point that the defendant is not entitled to raise on appeal. As a general rule, this applies to issues that are not apparent in the record of the case itself (such as when an attorney fails to investigate a possible defense).
Any one of the following reasons, for example, may prohibit an appeal (and justify a writ):
The defense did not lodge a timely objection at the time of the alleged injustice (but should have).
A final judgment has not yet been entered in the trial court, but the party seeking the writ needs relief at once to prevent an injustice or unnecessary expense.
The matter is urgent. (Writs are heard more quickly than appeals, so defendants who feel wronged by actions of the trial judge may need to take a writ to obtain an early review by a higher court.)
The defendant has already lodged an unsuccessful appeal (defendants may file multiple writs but the right to appeal is limited to one). But filing a writ that simply mimics an unsuccessful appeal is a frivolous writ and will be dismissed immediately.

What is a writ of habeas corpus?
Defendants who want to challenge the legality of their imprisonment -- or the conditions in which they are being imprisoned -- may seek help from a court by filing an application for what is known as a "writ of habeas corpus."
A writ of habeas corpus (literally to "produce the body") is a court order to a person (prison warden) or agency (institution) holding someone in custody to deliver the imprisoned individual to the court issuing the order. Many state constitutions provide for writs of habeas corpus, as does the U.S. Constitution, which specifically forbids the government from suspending writ proceedings except in extraordinary times -- such as war.
Known as "the Great Writ," habeas corpus gives citizens the power to get help from courts to keep government and any other institutions that may imprison people in check. In many countries, police and military personnel, for example, may take people and lock them up for months -- even years -- without charging them, and those imprisoned have no avenue, no legal channel, by which to protest or challenge the imprisonment.
The writ of habeas corpus gives jailed suspects the right to ask an appellate judge to set them free or order an end to improper jail conditions, and thereby ensures that people in this country will not be held for long times in prison in violation of their rights. Of course, the right to ask for relief is not the same as the right to get relief; courts are very stingy with their writs.
Potential