What do I make of unlinked tables in the same database? Just a basic question here but it's been bothering me for a while. Do all tables have to be related to another? I'm trying to design a database so far I have 15 tables, 13 of them have relationships with each other creating a regular snowflake pattern, but I have two other tables that only relate to each other, meaning they don't have any relationship to the any of the other 13 tables. Does this mean I should review  my design or is this considered OK? 

Thank you for your help in advance.
No, there is no requirement for tables to be related to one-another.  The fact that you do have two tables separate from the other thirteen would be a code smell and you should be sure that you have not forced two different business objectives into a single database.  If the two really do support genuine use cases for the others then there's no problem.

A typical example are "staging" tables.  Here data from an external source is loaded into a stand-alone table.  Then that data is validated.  Only then is it copied across to inter-related tables.  Another is to hold system configuration or user options.  These have no foreign keys to the "business" data but support the operation of the system which uses that business data.

Lastly I would point out that the logical and physical designs can be different.  The data model (logical) may document foreign keys but the data base (physical) does not declare foreign key constraints for one reason or another.  This would be a very specific choice for particular circumstances and relatively rare, however.