A quick guide:
Under the criminal justice system of England and Wales there are two modes of trial: Summary – that is before the Magistrates - and Indictment – that is, a jury trial before the Crown Court. For the majority of offences, the mode of trial is prescribed by the law. For less serious offences such as common assault, driving without insurance and the like, then the defendant can expect to find themselves before the local magistrates. For very serious offences such as murder, robbery, rape and so on, the law requires the defendant to be tried on indictment. There is also a class of offences known as 'either-way' in which the defendant may elect to be tried in the Magistrates' or the Crown Court. This is for mid-level crimes like theft or assault occasioning actual bodily harm.
There are two main differences between the modes of trial. The first,is that the Magistrates do not have a jury where as the Crown Court does. The second difference however is sentencing power; the Magistrates in Summary Only cases are only able to sentence someone for up to six months imprisonment or a £5,000 fine, and even on Either Way cases may only sentence for up to an aggregate of one year's imprisonment (up to six months per offence) or a £5,000 fine per offence. The Crown Court's sentencing powers are limited only by the law creating the offence – they can impose unlimited fines and life terms in prison unless forbidden by statute.
This creates a tactical situation for a person awaiting trial on an either-way case. If the case against the defendant is a strong one, they may decide to be tried in the Magistrates' Court and so only incur a lesser sentence. A Crown Court judge may take a very dim view of a defendant wasting the court's time on a hopeless case and sentence accordingly. On the other hand, Magistrates, whilst obliged to be an unbiased tribunal do see a large number of trials and are less likely to be susceptible to flourishes of advocacy that might persuade a less experienced juror. I vaguely recall some statistics that suggest up to sixty percent of jury trials result in an acquittal compared to something like ten percent of those in the Magistrates! Inevitably then a good case will go before the Crown Court. It should also be noted that the Magistrates may refuse to accept jurisdiction over a case and commit it to the Crown court regardless.
A defendant on a serious either-way case who elects to be tried by the Magistrates is not necessarily getting off lightly. Under these circumstances, if the Magistrates do not consider themselves to have sufficient sentencing powers, they can commit the defendant to the Crown Court for sentence. All may not be lost however; Crown Court judges are used to dealing with more serious cases than the Magistrates and may, having considered the Defendant's case, particularly if there are mitigating factors, actually sentence less harshly than a shocked and appalled Magistrate would have.