The basic difference between testate and intestate succession lies in having or not having a legally valid will. 
During a person’s lifetime he or she has absolute control of his or her estate and can therefore exercise any intention they wish to. However, after death the person no longer has this ability and therefore must, in order to prevent fraud and to ensure that his or her wishes are carried out, record his or her wishes (in writing) before death. This may be done by way of executing (signing) a legally valid will. Thus leaving verbal instructions or having the intent to execute a will, but never actually doing so, is insufficient for these purposes.
However, where a will is poorly drafted or not executed with the assistance of a professional, it may be unenforceable and / or legally invalid, which I turn will result in the estate being administered intestate – as if you never executed a will in the first place!
