Not to be confused with hardy geraniums, there are a large number of pelargonium species, about 250, but few of these are grown as it is the cultivars that are popular.
There are three groups of these. The zonal which are upright and have simple flowers in dense flower heads, the regal which are also upright but with more fancy flowers, the ivy leaved which have shiny leaves and trail and the scented leaved which have relatively insignificant flowers but fragrant foliage. All flower forms can be either single or double.
Although they can all be used as bedding and in containers they tend to be used in slightly different situations: the ivy-leaved for example are used in hanging baskets and window boxes where hanging plants are needed.
None are hardy in the UK (they originate from South Africa) so they must not be planted out before the frosts have passed. They will grow in a wide variety of soils and are reasonably drought tolerant. It is best to start them from cuttings each year taken in late summer or autumn and over winter these as young plants, discarding the old plants after use.
They were once considered part of the hardy geranium genus, hence the confusing use of the name of geranium that is still used to describe them.