Posting groups in Microsoft Dynamics NAV are very important part of configuration process. All G/L Entries depends of correctly configured posting groups.
We can find a lot of them: General Business and Product Posting Groups, VAT Business and Product Posting Groups, Customer and Vendor Posting Groups, Inventory Posting Groups…
Sometime, it is very confusing how it works, especially for unexperienced people.
You can find good explanation on same in this post by Totovic.
Nicely explained, you should see it here. I feel must to see this post for every Navision Consultants either fresher or experienced.