Not only NoSQL is not ACID compliant by design. NoSQL movement embrace the BASE (Basically Available, Soft state, Eventual consistency) claimed to be the opposite of ACID.
NoSQL database are often called Eventually-Consisted database.
To understand the differences you should drill down into the CAP theorem (aka Brewer's theorem)
Visit http://www.julianbrowne.com/article/viewer/brewers-cap-theorem
