A Will is a written document, in a specific format, which is designed to carry out your wishes in regards to your assets after your death. It can also provide instructions to your Executor/s on other matters, such as selecting guardians for your infant children, and your funeral/burial arrangements.
A Will only comes into effect after your death. It has no power while you are still alive. You can change your Will as many times as you like during your lifetime. Only the last valid Will you make before you die will be used to reflect your final wishes.