For the purposes of a will the big difference is that only those organisations accepted as charities by the Inland Revenue are exempt from inheritance tax. This distinction is therefore only likely to bother you if you are planning a charitable gift as part of tax planning.
You may if you wish leave your entire estate to your executors to allocate between charities of their choosing.
This course of action is not recommended as your executors will quickly be inundated with literature and begging letters from hundreds of charities who have been made aware of the terms of the will.
On this point I speak from personal experience. Within days of publication of the will both my office email and the executors' were inundated with spam brochures and mails from dozens of charities.
Sifting through them to establish some criteria for final selection was a huge task. Fortunately both executors agreed as to the criteria to adopt but what if they had had different views as to what the testator would have agreed with?
It is wise to specify what is to happen to the money if the chosen charity has ceased to exist or has changed its name or merged with another.
Clarity is again the order of the day. It is a true but regrettable fact that charities can act in a most uncharitable manner when large sums of money are at stake. In this they are no different from any other class of beneficiary.
Given that a gift to a registered charity is tax-exempt it can be a last resort weapon in tax planning. If you have done everything you can to avoid inheritance tax and there is still a potential bill, leave the money to charity.
Your relatives will still miss out on the amount in question but at least you are deciding where the money should go rather than leaving it to be paid into the general taxation pot.
If you are keen on benefiting charitable causes you may be interested in a couple of schemes operated by solicitors in conjunction with charities. The details are given in the Appendix.