Basically, it's finding a virus on the basis of speculation - the virus scanner believes it to be a virus, but it can't be 100% sure unless there is signature files available. This is the problem, although it happens rarely, NOD32 and other av companies that use Heuristics will often find files that are OK as infected, which causes a lot of stress.
I think that the only time you would need heuristics is if you owned a large company, that did millions of transactions a day - where your chances of being hit by new viruses was quite high. For home users, it's really not.