Just a bit more info: They are only held where the defendant has pleaded "not guilty" to an either way" offence. These are offences which can either be heard at the Magistrates' Court or at the Crown court. After the decision has been taken that the matter should go to the Crown Court papers have to be prepared and served upon the defence before the matter is formally "committed" to the Crown Court.
Although they begin their life at the Magistrates' Court, serious offences which can only be heard at the Crown Court (which are called "indictable only" offences) do not undergo a formal committal. They are immediately "sent" to the Crown Court (usually at the first hearing).
There is a pilot test going on at the moment in one area (not sure where) to do away with committal hearings. The plan is that once it has been decided that the case should go to the Crown Court it is immediately sent there in the same way as indictable only offences.