It's like swinging your dick around, only it's overexaggerated and much sharper. Also better looking.
The Longsword is basically the Greatsword, but taking speed over power and having faster albeit less combo ability. They also have more elemental power than greatswords, but less raw power.
To make up with this, they've got around the same sharpness as a Greatsword, if not better.
Every time you land a hit with a longsword, the animu fightan' gaems gauge(Spirit Gauge) fills up slightly.
The gauge can be consumed by unleashing your supa supa weeaboo slash combo of death, gaining ESP and basically becoming a blender of pain(though not as blendery as Dual Swords) to tear your Kut Ku Adversary a new one.
Filling the gauge completely will also give you an attack boost for a short while. You can roll out of combos with a Longsword.
A Longsword is a very useful weapon for beginners and experts alike, and some of the best weapons in the game are Longswords (Smoulder Dragonsword and Devil Slicer come to mind.)
Prepare for some weeaboo slashan jokes from us, though.