Sorry trevor, assumed you knew. I'll try to keep this brief....
Planning applications are, in theory at least, the responsibility of a committee of councillors chosen by the Council to decide planning matters. However they usually make up a list of application types or conditions that mean officers can deal with the applications (a scheme of delegations and usually covers house extensions, small scale new housing, shop advert setc) with a list of reserved items that are always determined by the committee (often bigger housing schemes, green belt sites etc). Councillors then usually have some other conditions that might make an application go to commitee (eg more than a certain number of objections) and they also reserve the right to bring any other application to committee for any reason they choose (known as a 'call in').
The exact procedure varies from council to council, but basically a 'delegated' item is dealt with by officers behind closed doors. A case officer will make a recommendation and a planning manager makes the final decision and sends out the formal notice. Only written comments are taken into account. Anything heard by the committee will go to a public meeting where, usually, a case officer explains the basic facts of the case, the applicant speaks in support of their application and any objectors can speak to explain their concerns, then the councillors can ask any of them questions. The Councillors then have a public debate about the merits of the proposal and make their decision at the meeting.
There are obviously pros and cons to both procedures but it is common for anything with a lot of objections to go to the committee because the case has had a very public airing and everyone knows the ins and outs of why the decision was made.
Hope that made sense?