Disfellowshipping is the result of breaking Watchtower Rules. Three Elders invite you to judicial committee meeting to ask you some questions or confront you with some evidence that you may have broken some rules. Then they decide if you should be removed from the organization or not.
Leaders of the Watchtower say, "disfellowshipping is an action taken by a judicial committee against an unrepentant wrongdoer, disassociation is an action taken by an individual who no longer desires to be one of Jehovah's Witnesses. ... Since disassociation is an action taken by the publisher rather than the committee, there is no arrangement for an appeal." Shepherd the Flock of God, page 110 to 112
In this way the organization can tell the world they don't take action against JW's for leaving the organization or having a blood transfusion. Instead a JW disfellowships themselves by breaking certain Watchtower laws. 
When you do things the organization does not approve of, you indicate that you no longer want to be one of Jehovah's Witnesses. The end result of disassociation is the same as disfellowshipping.
Whether you get disfellowshipped or you disassociate yourself, an announcement is made to the congregation that you are no longer one of Jehovah's Witnesses. Jehovah's Witnesses are not told whether you were disfellowshipped or you disassociatied yourself. This is the announcement that is made to the congregation. "[Name of person] is no longer one of
Regardless if you disfellowship yourself (disassociation) or the Elders disfellowshipped you, all JW's must shun you, including your family (with special exceptions)
Here's what the official rule book of Jehovah's Witnesses says about disassociation. This information is only available to Elders.
"Actions that may indicate disassociation indude the following:
Making known a firm decision to be known no longer as one of Jehovah's Witnesses, ... Joining another religious organization, ... Willingly and unrepentantly taking blood, ...Taking a course contrary to the neutral position of the Christian congregation" "Shepherd the Flock ofGod"-l Peter 5:2, p 110 -112
1. Since 1981 it has been a sin to leave the organization. Here's what they say about JW's who decide to change their religion or believe something else. 16 "Persons who make themselves “not of our sort” by deliberately rejecting the faith and beliefs of Jehovah’s Witnesses should appropriately be viewed and treated as are those who have been disfellowshipped for wrongdoing." Watchtower magazine, Sept 15, 1981, p 23, par 16 See Persecution of Jehovah's Witnesses. Freedom of Thought, Conscience, and Religion