I've gotten a ton of questions about this so I thought I'd post to help clarify things.
Our son Roman has Hypotonia and Sensory Integration Disorder - Hypotonia means low muscle tone. This means that his the relaxed state of the muscle is much more relaxed than someone with standard muscle tone. So this means that he has to work much harder to do anything involving large and fine motor. He can strengthen the muscles he has, but the relaxed state will in all likelihood remain as relaxed as it is now.
His case is moderate, he can walk and now jump (yay!), but he is in physical and occupational therapy twice a week to help.
Also, Hypotonia is usually coupled with a much larger diagnosis, like Cerebral Palsy or Autism. It's almost like saying you have a fever. Well what's causing the fever? We have no clue. We've gone through genetic testing and most recently an MRI, and everything came back completely normal, so our son is in the small category of kids with Hypotonia with no main diagnosis.
Sensory Integration Disorder is best described with an example: Person "A" walks into an auditorium and is able to hear people talking around him, but isn't bothered by it. It's a bit warm so he takes off his coat and waits for the game to start. Person "B" walks into the same auditorium and is so overwhelmed and bothered by everyone talking around him that he can barely concentrate on walking to his seat. The elevated temperature in the room makes him feel faint.
Person "B" has Sensory Integration Disorder. Now it can also go the other way, some people are sensory seeking rather than sensory avoiding - for example some kiddos with SID (not SIDS) hate the feel of sand on their bare feet, but some crave it. Some have an extremely high pain tolerance and some scream bloody murder at a simple papercut.
With the right kind of therapy, these kiddos can flourish. I've seen it first hand.