This is an order that a person is kept in prison for a specified, and definite period of time.
The minimum period of a prison sentence in the Magistrates Court is 5 days. The maximum sentence that a Magistrates’ Court can pass is one of 6 months imprisonment (unless there are two either way matters, in which the court can pass a sentence of up to 12 months)
There is no minimum sentence in the Crown Court and, in theory, no maximum (other than the maximum sentence for the offence) although in practice sentences of over 20 years are very unusual (and reserved almost exclusively for large scale drug importation).
The rules surrounding exactly when people will be released are very complex, but in essence, you will serve half the prison sentence actually in custody (in the prison) and then be released. The remainder of the prison sentence will be spent supervised in the community under the probation service.
If you break the terms set by the probation service, they you can be recalled to prison (unless this is because of a new offence, there will usually be a warning given first). If that happens, then you will be arrested and taken back to prison.
There are different sorts of recall, sometimes it will be for a fixed term of 28 days, sometimes to serve the amount of time from when the ‘notice of recall’ is issued to when your sentence would have expired. In either case, you can apply to the Parole Board to be released.
Sometimes you may spend more or less time than half the stated sentence in prison. If you misbehave in prison you can be ‘charged’ with an offence and, if it is found proved, have extra days added to the sentence.
Sometimes, people are released earlier on an electronic tag. Again, the rules are complicated, and it depends on the length of sentence and type of offence that lead to the prison sentence, but you have to have served six weeks and at least a quarter of the sentence and you can be released 135 days earlier.