Innocent means that you did not commit the crime.
Not Guilty means that there was not enough evidence to determine that you did commit the crime.
There is a big difference between the two. In the US, we find defendants not guilty; this means that there wasn't enough evidence to detemine, beyond a reasonable doubt, that you did infact commit the crime. It doesn't mean your innocent (you still may very well have done it) it just means that your guilt couldn't be proven.