Seals were once widely used as a method of authenticating documents (particularly important in societies where the majority of the population were illiterate) and it was therefore common and one time to find that all persons of any importance would possess a personal seal. The seal was itself a metal matrix or die used to form an impression in beeswax or wafers of flour paste which had either been directly applied to the document or impressed separately and attached to the document by means a strip of parchment or other material such as silk.
The Great Seal of England was (and is) simply the monarch's seal, first used by Edward the Confessor and used ever since to sanction all royal decisions; including such things as appointments to office and foreign treaties as well as public acts of legislation.
Normally kept in the custody of the Lord Chancellor, each monarch would have their own seal (and sometimes more than one depending on the length of their reign). William I adopted a double-sided seal featuring himself crowned and enthroned on one side and mounted on the other; a practice followed by all monarchs that succeeded him as the design embodied the two supposed facets of kingship: on the one hand as a dispenser of justice and on the other as a military leader leading the nation into battle. (The one exception to this would be the Great seal adopted in 1649. As England was then formally a republic, the seal features a representation of the House of Commons itself.)
The Great Seal is still in use today and is used to formally seal all Acts of Parliament to signify that they have received the Royal Assent and now form part of the law of the land.
At www.pro.gov.uk/virtualmuseum/millennium/greatseal/greatseal/default.htm you can see as an example details of the second Great Seal of Elizabeth I.