Can someone explain the APPS schema part of the TCA structure

I am trying understand the structure of TCA.  I have been informed that TCA is all under the APPS schema, but when I look at the HZ_PARTIES table for example in APPS and look at the same table named in AR, everything looks exactly the same. 

What exactly does the APPS schema do?  

If I was to draw a diagram trying to show AR and AP 'relationship' on tables to the TCA structure, I am told there is only one set of tables that exists in AR.

IF that is true then what is the APPS for? 

Where is the structure of TCA actually 'exist'?