None-Natural link building = Done by software or auto tools. You could get 100-100,000 links in a month.
(100,000 / 30 days = 3333 links pr day! Is that natural? Could you do that your self pr day by hand, by "submitting" one and one link? No way!
3333 links pr day could only be natural if you are a well established authority site, very big authority site - youtube, newspapers and so on...)
Natural link building = Google sees that you get a steady flow of links over time. So big spikes in your incoming linking is not good.
E.g. Do not get 500 links every monday. Spread them out over the week, 70 pr day. I think even 70 new incoming links would be to much, but I'm only guessing here...
Think about it; Your new domain just registerd. A week after this you start to get 70 links each day? And you are for sure no authority site.
For a commercial website you just created to sell something or promote something,... personally I think 2-10 links pr week is natural. If you keep that up for a long time, you will probably get bether PR and all the good stuff that comes with it...
Auto tools like SEnuke will just boost you in the beginning, but destroy your site very quickly WHEN (not if) Google catches up...