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The Shorty Plus is the 'apochromatic' version. The normal Shorty is a simple 2 element 'achromat'. The 'Plus' version also has a larger lens to reduce vignetting with wider angle eyepieces.
As Phil says, an 'apochromatic' lens may contain one lens made of fluorite. However the term 'apochromatic' is really a qualitative one and doesn't prescribe any particular optical configuration. The Shorty Plus barlow uses 3 elements of crown and flint glass to achieve its 'apochromatic' status, rather than using fluorite. Neither system is necessarily any better than the other, as far as I know.
The new Meade 5000 barlows employ 4 elements! (at a price!)
I don't know what the Tele Vue PowerMates use...
It all comes down to trying to bring all colours of the spectrum to the same focus, avoiding 'chromatic abberation' or colour fringing around bright objects, which is an inherant property of any simple, single lens.
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