Hi
It depends on what you are trying to do,
If you only want to read information you can do so by connecting directly to the SQL database, this can use SQL or ODBC drivers, so whatever suites however if possible you should use the direct SQL driver as it is faster
If you want to read and write you should never write directly into the NAV tables, being a financial system there are certain business rules and checks and balances that are made against all incoming transaction entered into the NAV system, if you write directly to the SQL tables these are missed and all sorts of things will break
To handle this correctly you have several options, the best option is as Miguel says, web services, these will provide the most flexibility and control over the process from the remote application, but you can also have the two systems communicate through a shared SQL table (Specially created for this purpose), files (XML, CSV, etc....), NAV can connect to TCP ports, and oter methods
Cheers
Nev