Here's an overview of the major types you'll find at nurseries.
Common or zonal geraniums
Zonal types, with round leaves topped by umbrella-shaped flowerheads, are the plants most commonly thought of as geraniums. "Zonal" refers to a zone or ring of deeper color just inside the leaf margin (the zone isn't always conspicuous).
There are single-flowering types, but doubles are the big sellers. Colors include red, magenta, salmon, orange, pink, lavender, and white; some flowers have a second color.
A subgroup of zonals, called fancy-leafed geraniums, are grown primarily for their variegated leaves. A good example is 'Mr. Henry Cox', which has yellow leaves with a red zone and splashes of green and purple-brown.
Standard zonals like full sun; variegated fancy-leafed types do best in partial shade. Use them as bedding plants or grow them in pots.
Their succulent, glossy leaves resemble those of ivy. They produce single or double flowers in a range of colors - red, pink, lavender, white, and bicolors - over an extended period. Because of their trailing habit, floriferous nature, and tolerance of tough conditions, ivy geraniums are one of the most popular summer annuals for containers. The Balcon series is particularly carefree - they're self-cleaning. In frost-free climates, ivy geraniums are also used as a ground cover. Ivy types normally perform best in full sun; in very hot climates, like the deserts, they appreciate afternoon shade.
This group is prized for the delightful fragrances its leaves release when brushed or rubbed. Rose, lemon, and mint are the predominant scents, but there are many more, including apple, coconut, ginger, nutmeg, and other herbal aromas. Leaves vary greatly in size and shape; some look more like leaves of oaks, grapes, or ferns than geraniums. Leaf textures can be coarse or smooth as velvet. White or soft yellow variegation is common; a few kinds have brown splotches.
All scented types are attractive in containers, but the trailing kinds like chocolate mint, peppermint, and 'Snowflake Rose' are ideal in hanging baskets or window boxes.
In frost-free climates, pelargoniums make great landscaping plants. Try apple or nutmeg as edgers, advises Kathryn Jennings of Katie's Scenteds, a geranium specialist in Lakewood, California.
Most scented types look their best when given some shade "at least in the afternoon, and especially in hot climates," says Jennings.