All shrubs can be pruned to some degree, to shape them or to keep them smaller than they would naturally grow. However, some shrubs are pruned annually to produce flowering stems.
Some shrubs, like Buddleia Daviddii (The Butterfly Bush) flower in late summer on wood made during the same season. They make growths from buds that have been resting over winter, so you need to be careful when pruning that you do not damage these buds. These shrubs are cut back in early spring when the heavy frosts are over. You start by cutting out any wood damaged by frosts, then to increase the size of the flowers, all shoots are pruned back hard.
Shrubs such a Ribes (Flowering Current) and Forsythia, should be cut back immediately after flowering. This encourages the shrub to produce long shoots. The longer the shoots, the more flowers you get.
Others like heathers (Erica) and Lavenders (Lavendula) are pruned immediately after flowering by cutting the flowering heads off with a pair of shears. This is to stop them producing seed. It also keeps them tidy.
Shrubs like Rhododendrum do not really need pruning. They grow naturally in a good shape. These need dead-heading. This is simply removing the dead flower heads.This increases the flower yield the following year.
There’s no magic to pruning, don’t be afraid to do it. If you cut too much off one year, it will grow back the following year. If you prune at the wrong time, trhe shrub may not flower so well (or not at all) the following year but the year after that, it will be fine.