Common, zonal or "horseshoe" pelargonium: The Matisse geranium, with voluptuous, banded leaves in the shape of torn circles. Ecstatic blooms come in every color but really should be lipstick red. Annuals in most of the country, shrubs in California, where succulent stems need clearing out every season. Some summer shade in ferocious heat. Can take seasonal water or stand irrigated spots. Breeders produce new hybrids every year. Check plant sticks carefully. Some breeds will remain compact window box plants, others can become bushes.
Regal (or Martha Washington) geraniums: These produce fancy foliage, what Barton describes as a "cucumber-type leaf." Every color, azalea-type blooms, whose blotchy throats, or nectar pads, evolved to attract insects. Hummingbirds appreciate them. Crossed with ivy geraniums for the fancier new balcony plants. Improved soil will produce yet more blooms.
Ivy geraniums (Pelargonium peltatum): They break down into dozens of cultivars that can have the shape of ivy. Can trail from 18 inches to 5 feet, create showers of blooms. Balcony plants, ground cover. Primary and pastel colors, it's got them. Stripes, ditto. Single blooms, double blooms. Just don't overload the balcony.