Which parts are activating itself.
What is the difference between autoimmune problems, it all causes pain, stiffness, inflammation, fatigue, brain fog etc. You can have1,2,3 or all of the symptoms from when you first see the doctor.
I don't mean different sub types, i mean as in rheumatoid, psoriatic, Lupus.
Is it different parts of the immune system activating itself in rheumatoid and another part
activating in psoriatic.
Also can some people get rheumatoid inflammatory arthritis and others not get the inflammation.
Rheumatoid Arthritis and Psoriatic arthritis and Lupus are all autoimmune diseases but there are so many different subtypes and variants that you can literally have all three at once all acting on different parts of the immune system OR you can have someone with Lupus and someone with RA who's body is being attacked by the same part of the immune system. It's a huge, complicated, mess, and it's party why you need a good rheumatologist with experience who can judge exactly what it is you have and where to start. In a nutshell, though, the main answer to your question is that the different is what symptoms your chronic illness is producing that determines what it is probably classed as, rather then what part of the immune system is acting up. Hope that helps some! Keep on keepin' on, DPM 
in addition to the wonderful answer gave, it can also make a big difference when it comes to treatment decisions. as we advance more and more into personalized medicine, we are seeing less general use of rheumatological medications and more that are specialized and directed at specific sub-variants.
