In functional test type may test all functionality of 
application.that means application should be correctly or 
not.
     navWhen testing links from one topic to another, many 
authoring tools can help by providing a report of all links 
with both the source and the destination topics listed. 
This may be enough for preliminary testing, but a thorough 
test should be done before shipping by actually following 
every single link in the system. This is especially 
important for any links outside of the system, such as web 
pages, as these can change overnight.

igation

In functional testing we test the functionalty of the 
application like whether all the given features are working 
according to the customer requirements.

navigation testing is like testing the page navigation of 
application.
For example clicking on login button of any application 
should navigate to the home page.

Navigation Testing is essentially a type of Functional Testing, since the page navigation is also a part of the application functionality. It is of course not uncommon to have no Navigation testing, as a separate activity, and to cover it as a part of functional testing.

Navigation testing as the name suggests, is to concentrate solely on whether navigation in the application is designed as expected.

Functional Testing in addition to Navigation testing, also tests all the other areas of functionality.