In English Law a writ was one of two things.
In mediaeval times onwards a claim could only be brought before the court if the circumstances which had occurred exactly matched the relief the court could give. Thus there was a writ of ejectment (for getting people off land) and a writ of certiorari for quashing official verdicts. This system was restrictive, and intentially so. Giving the courts more powers would give the peasants more powers, which was not popular.
In more recent times (under the Rules of the Supreme Court, most recently 1981) a writ was the document with which a court case was commenced in the High Court of England and Wales (as opposed to a 'summons' in the County Court). The writ set out the names of the parties and the actions complained of together with the 'prayer' (the request for a remedy, such as damages).
Following the plain english ethos of the Woolfe reforms[?] the Civil Proceedure Rules[?] did away with both writs and summonses and replaced them both with the 'Claim Form', which simply is the form to fill in if you want to make a claim...
