Many theories have evolved through the years regarding the aims of sentencing and the aims of sentencers when sentencing. The phrase (the theories or in other words the aim or the purpose of the criminal law) may sound a convenient one but it is prone to be misleading. The present criminal law, with its myriad offences, is not a coherent body of law seeking to achieve one particular aim. Mainly school of thoughts suggested that the real aim of the criminal was to provide an organised means for controlling the passion of revenge.
 Modern treatments of the subject usually list the four competing theories of punishment as those of retribution, deterrence, incapacitation and reformation. By retribution is meant of course, retaliation, or revenge against the offender for committing a wrong.  Under retributive theory, they views a crime as a wrong which its very nature justifies the infliction upon the criminal of a certain amount of punishment.
Strictly speaking, the retributive theory of punishment requires that punishment be inflicted even though it would serve no apparently useful purpose. It’s more to the application of the principle “an eye for an eye, tooth for a tooth”. For deterrent theory, suits with the name, use to deter people from committing the same crime in the future. Its aim is to punish the wrongdoer so that he repents and will not commit an offence again.
It hopes that future offenders will be deterred by seeing the punishment meted out on particular offender. Deterrence can be divided into two parts which is general deterrence that deter others from committing similar acts and specific deterrence that deter him (the offender) from committing similar acts in the future.  The next one is the reformation theory or by its other name, rehabilitative theory. This theory sees crime as a social disease, for the cure of which the steps can and should be taken.
The punishment, according to this theory, is inflicted for the purpose of reforming the criminal and inducing him to lead a non-criminal life in the future.  The difference between rehabilitation theory and deterrent theory is that rehabilitation theory was designed to change an evil person into a good person, while deterrent punishment is not intended to change the criminal ‘s general outlook, but merely to stop him from pursuing his evil ways and putting his outlook into practices in the future.
 The other theory is the incapacitation theory. This theory which also known as the restraint justification is that the criminal justice system, through its capacity to imprison dangerous offenders, thereby protects society from the commission of further crimes by these offenders.  It provides a degree of public protection by exerting a general deterrent effect on potential offenders and by keeping some offenders out of circulation, i. e. for any period when they are incarcerated.