Stop your opponent from causing a nuisance. Stop your opponent interfering with your possessions. More recently the courts have also allowed orders to exclude opponents from coming within a certain distance of the applicant's home or place of work. You cannot get a power of arrest with this type of injunction. If your opponent breaks the injunction you will need to go back to court to get an order to send your opponent to prison.
Harassment includes, nuisance phone calls, stalking, threats, excessive noise etc.
The harassment has to have happened more than once. In the case of stalking you must show the behaviour caused you to think that person was likely to use violence on you. The Act can be used to prevent harassing behaviour by neighbours, protesters, family members, the media etc. If a person is guilty of harassment the police have powers to arrest them and sentence them in either the Magistrates Court or the Crown Court. In the Crown Court they can be sentenced to up to 5 years in prison or an unlimited fine as well as an order preventing them from harassing you.