The Will combines the role of executor and trustee. In other words they are the same person(s). An executor is the person(s) who administers your estate after your death. They are charged with the responsibility of paying all creditors and ensuring that your wishes and instructions are carried out.
It is usual for the main beneficiary to be the executor of the estate with an alternate being chosen in case the executor predeceases you, or is unwilling or incapable of acting. You can choose up to 3 executors in your Will.