You appoint an Executor in your Will. It is strongly recommended that you appoint alternate(s) in case the first person that you named is unable to act as executor. You can also appoint more than one Executor to act together.
The Executor is the person who you entrust to carry out your instructions in your Will. Their duties include such things as finding your will, arranging your funeral, gathering and securing your assets, paying your debts, income tax and other expenses, arrange for Probate (if needed), and distributing your assets according to the terms of the Will.
If someone was to contest the Will or file a claim against the Estate, the Executor would act as a defendant in this action. If you choose a family member or friend, you should ensure that they are trustworthy, can handle paperwork and can work with the beneficiaries. Often a Will allows the executor to hire professional help and pay for that out of the Estate. You can also hire a professional to do this job, but that professional will charge a fee for this service.