The normal distribution is the most useful, adaptable, flexible, used statistical distribution there is. All other distributions are, in a sense, subsumed by it because of the Central Limit Theorem, which says that in some circumstances any statistical distribution can be modelled by a normal distribution.
The probability of any occurrence in the rangetoisifis small.
The Normal distribution is writtenwhereis the mean andis the standard deviation. It has three characteristics:

It is symmetric about the mean
\[\mu\]. 
Virtually all occurrences occur within three standard deviations of the mean.

The normal distribution is bell shaped.
There are some limitations for the normal which means it cannot be a possible exact distribution for certain quantities, even though it may be a good approximation. For example, the normal allows negative occurrences, but negative lengths are not possible, so it cannot be a possible distribution for lengths of worms.