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There are A, B and C shares and a few other flavors out there. The basic idea behind B-shares is that the buyer of the mutual fund pays his fees only when he sells the mutual fund. The advantage in part is that the commission he would have paid up front is now deferred - he pays it on a bigger lump of assets but he got to compound all of that time without the load, which came ahead of his investment.
The technical jargon around this name is "contingent deferred sales charge".