What's the difference between a hand-and-a-half sword and a longsword?

I ask because when browsing Kult of Athena and other sword places, swords sometimes called "hand-and-a-half" will have a grip length of 7-8 inches while a longsword will have a grip length of 6 to 7 inches. Furthermore, the two different grip lengths frequently are interchanged, so there is no set grip.

Additionally, most longswords I've encountered are optimized towards two handed wielding, yet I've also heard that you can use them with one hand. What gives?