Streptocarpella is a subgenus of the genus Streptocarpus*.  The flowers and seed pods are very similar to "regular" Streptocarpus, but the leaves and stems are very different .  However, they are grown in basically the same conditions- perhaps tolerating hotter conditions, and cuttings are easy.  They are sometimes called 'nodding violet'.
Genus (Subgenus):    
Streptocarpus (Streptocarpella) = Streptocarpella

Streptocarpus (Streptocarpus) = "regular" Streptocarpus
Streptocarpella are "caulescent", which means that they have stems (Latin caulis = stem/stalk).  Whereas, regular Streptocarpus don't really have stems.
Streptocarpellas generally are clump-forming or trailing plants.  When they are young, they can be upright.  Their flowers are only about 2.5-3.5cm, and their colour range seems to be limited to purple, pale pink/purple, and white.
Streptocarpellas and regular Streptocarpus do not interbreed. 
From what I have seen, Streptocarpella leaves can be decussate in arrangement (each pair of leaves at a node is at 90 degrees to the ones preceding or following it), or ternate (whorls of 3 leaves at each node).  However, I have a specimen that exhibits both on the same plant!
Close-up of ternate leaf arrangement.
Streptocarpella flower and bud
A Streptocarpella in a hanging basket
Streptocarpella stem-tip showing decussate leaf arrangement
* Studies have shown that African Violets, genetically, should be nested within the Streptocarpella subgenus. So, that just makes things messy. African Violets currently have their own genus "Saintpaulia".
Anderson's Streptocarpus,
28 Oct 2011, 19:13