Inheritance Tax is based upon the TOTAL value of the estate. So EVERYTHING is added up, including the value of properties, bank savings, shares, National Savings certificates, ISAs, cars, jewellery, furniture, etc, etc.
If that total comes to over £325,000, Inheritance Tax is charged at 40% (or possibly at 36% where there's a charitable donation) on the excess. If the estate is left by a person who has previously inherited from their spouse or civil partner, the exemption can be doubled to £650,000 because the spouse's/partner's exemption is also inherited.
If the TOTAL value of the estate comes to less than £325,000 (or £650,000, as appropriate), no Inheritance Tax is payable.
http://www.hmrc.gov.uk/inheritancetax/intro/basics.htm