How to use ROWGUIDs

I own a small retail organization and our POS system has a SQL based backend.  As you might guess the company that does our POS system is not very helpful when we need custom work so I have been given read access to the databases and over the years  I have done some work with Access to build some systems that do reports and query tables that are not available out of the box. 

In order to do this, I have had to reverse engineer the table relationships (because the company will not share their schema) but as the number of our SKU’s increase  it is getting harder and harder (now over 27,000 inventory items)

One thing I have noticed is all of the tables have a rowguid field.  Knowing very little about SQL, I have done some research on what ROW GUID’s are  so I think I understand that they are a database wide unique identifier so you cannot have any duplicates  across tables – if that is true it Makes sense...  If not true can someone explain to me what they are

My questions are

• how do you use ROWGUID’s in Practice , since each table in database by definition has unique value in the ROWGUID field.  Since they are unique how do you know a record in table “a” relates to another record in table “b” ?  Today, many of the tables I use for example,  have a field called  “SKU” that is common field so doing a join on SKU is easy
  
  
• I am assuming that if I use the ROW GUID field in my query it would be faster.  Is that true?
  
  
• Since it is up to me to reverse engineer the relationships, Does anyone have any suggestions what to look for to try to figure this out
  
  
• Is there any documentation on the practical use of ROW GUIDs that would help me understand and improve my queries  remembering that I only have read access to the tables. and I used Access to do most of my work not to mention I am a novice at this.

Sorry for such a Novice question. Just trying to find a way to make my Access queries faster

Thanks

Gary