Donald Tusk is the President of the European Council. He shouldn't be confused with the other six Presidents, Antonio Tajani who is President of the European Parliament, Joseph Muscat who is President of the Council of the EU (which is different to the European Council), Jean-Claude Juncker who is President of the European Commission, Koen Lenaerts who is President of the EU Court of Justice, Mario Draghi who is President of the European Central Bank or the impressively named Vitor Manuel da Silva Caldeira who is President of the European Court of Auditors (the body that have refused to sign off the EU's accounts for 21 years because of the level of fraud and incompetence). I'll save you counting - that's 7 presidents, all on the public payroll.
The letter triggers a process of negotiating an orderly exit from the EU and agreements on what our relationship with the EU will look like when we've left. This process can take up to two years but can be cut short by the UK unilaterally or extended with unanimous agreement of EU member states.
If no agreement is reached then we will revert to World Trade Organisation rules which is how most of the world does its trade with the EU. These rules won't, of course, apply to the 80% of our trade that is done with ourselves and will mean no change to the 9% or so of our trade that is done with the rest of the world.
The British government's plan is to adopt the entirety of EU law (known as the Aquis) into domestic law so they are free to amend and repeal it at will. They also plan to withdraw from the jurisdiction of the EU courts and restore parliamentary sovereignty. There is also an intention to agree reciprocal rights for EU citizens living in the UK and vice versa and to preserve the Common Travel Area with the Republic of Ireland which provided free movement between the UK and the Republic long before the EU existed.
There is too much opportunity for the British government to backslide and break its promise to deliver Brexit. With any agreements negotiated with the EU the devil will be in the detail and nobody is better at finding the devil in the detail than UKIP. This is why UKIP is more relevant than ever.