First, let's look at the overall aims of pruning; I'll add the details later.

ENSURE PLANT STAYS HEALTHY

1. Remove dead or diseased wood -
• prevents disease spread,
2. Remove branches that rub together -
• prevents bruising and disease entry,
3. Remove crowded branches -
• improves ventilation,
• prevents lodging of disease spores,
• reduces humid conditions that allow infections to invade,
4. Open the centre of the bush by cutting to out-pointing buds -
• allows light to reach lower leaves that otherwise become unproductive,
• maintains easy access for picking fruit,
5. Promote strong branches and remove weak branches -
• ensures branches can bear the weight of fruit,

MAINTAIN FLOWERING AND FRUITING

1. Prune at the correct time of year for the plant
• shrubs that flower in spring on the previous year's wood are cut back immediately after flowering - the new growth will support next year's flowers,
• shrubs that flower on current year's growth in summer can be trimmed in spring,
• if flowers are produced on both new and old wood then to extend the flowering season prune about one third in spring and leave the rest,
2. Maintain a good balance between flowers and fruit - extension growth and leaves,

SHAPE AND TRAIN FRUIT, SHRUBS AND TREES

1. Restrict size of young and vigorous growth,
2. Encourage thick or bushy growth,
3. Re-invigorate older branches,
4. Fruit can be trained to grow in narrow spaces, e.g. cordons, fans, espeliers, usually with the help of a framework,
5. Climbers can be trained to over trellise work forming arches and other shapes,
6. Topiary is the art of trimming shrubs into shapes,

Note some plants are slow growers and are better left alone. If in doubt don't cut it out.

More on pruning here.

More on Hedge Trimmers here.