Disfellowship = 3 elders determine that (A) you committed some sort of "serious sin" and (B) you aren't "repentant enough".
They announce to the congregation that you "are no longer one of Jehovah's Witnesses" which is the congregation's signal to start shunning you.
Disassociate = you write a letter stating you want to quit being a JW. Two or three elders will verify that you really did write the letter. (* see below)
They announce to the congregation that you "are no longer one of
Jehovah's Witnesses" which is the congregation's signal to start
shunning you.
As you can see, the end result is 100% identical - you are shunned by every JW everywhere.
* Note from above: "Disassociation" can also be [done to you]. There are certain "serious sins" that the JWs don't treat as "disfellowshipping offenses" (joining the army, taking a blood transfusion, etc.) because they fear retribution or loss of charity status from certain governments. If you commit a "disassociation offense" they will assign 2 or 3 elders to verify that you really did what you did. Afterwards, they announce to the congregation that you "are no longer one of
Jehovah's Witnesses" which is the congregation's signal to start
shunning you.