What makes a High Efficiency Speaker

Can someone explain the “high efficiency speaker” as opposed to a “regular speaker”?
Are there certain recognized parameters that a speaker has to meet?
If so what are those parameters.
And some pointers in what to look for in this type of speaker.
Who is the recognized leader in this field and which companies make them?

I would presume Wilson Whamm's are in there but I was more interested in a slightly lower priced item.

(I spent my lunch money on the new Buggatti)

Thanks