Answer:
A will or last will and testament is a dated, written document that is witnessed by two independent people who are not related to you and are not left anything in the will.
A will can be simply a hand written letter (witnessed as above) and will have the force of law on your death.
Normally however it is wise to get a solicitor (attorney) to draw up (write) the will for you so that your wishes are written clearly and unambiguously.