Difference between Hemp and MJ

Are there any scientists out there who could tell me the difference between Sativa, Indica, and Ruderalis? From my understanding, these are actually three different Species of cannabis, with Cannabis Sativa composed of Hemp, C. Indica is what we are all here for, and C. Ruderalis is wild and hardy ditchweed. However, I know that MJ also has both an Indica and Sativa version. So does that make it Cannabis Indica Sativa and Cannabis Indica Indica? Does this make sense to anyone else??

indica is one, sativa is one, ruderialis is one that is awful and then you have hemp, sativa is actually what we should all be here for

I'm pretty far from a scientist but from what I understand hemp is cannabis of any species that has been bread not for THC production but for fiber production, they want tall one main stemed plants with lots of space between the nodes. on top of that the plants are choped when the bds are just starting, before the fibers start hardening.
as for the different species of cannabis, alot of people deny them even existing. one of the main things that makes a species a species is that they cannot breed with eachother. they later changed this to they cannot reproduce and produce fertile offspring, sine tigers and lions can have babies and so can horses and mules.
from what I've read the only thing making cannabis sativa different from inidica is its growth pattern such as leaf structure plant size, flowering time and branch sructure. and how long they live.
this is just from what I've read I'm not 100% sure, but dont think anyone really is

sativa is a more creatie clear go and have fun high and indica is couch lock sleppy high

Is this true for the scientific classifications of the plants as well?

if your plant is 100 percent sativa (which pretty much no plant is anyore because of the cross breeding)it would grow really tall and have really tall thin,and fluffy buds,now if it was 100 percent indica it would maybe get 4-5 feet high but the buds are really short dense, and compact, but it breaks down really fat because of its compactibility,ruderallis dont even mess with that bull pucky its just wild weed from over the centuries,mainly grows in russia and over seas but the is some the grows in the southern states. so yes it could be classified in different classifications. just go to google.com and type in either sativa weed or indica and examine the pics , you'll see the difference(and theses are all different variations from the hemp plant which is marijuana just grown differently)