The shield is quartered, depicting in the first and fourth quarters the three passant guardant lions of England; in the second, the rampant lion and double tressure fleury-counter-fleury of Scotland; and in the third, a harp for Northern Ireland. The crest is a statant guardant lion wearing the imperial crown, himself on another representation of that crown. The dexter supporter is a likewise crowned English lion; the sinister, a Scottish unicorn. According to legend a free unicorn was considered a very dangerous beast; therefore the heraldic unicorn is chained,as were both supporting unicorns in the Royal coat of arms of Scotland. The coat features both the motto of English monarchs, Dieu et mon droit (God and my right), and the motto of the Order of the Garter, Honi soit qui mal y pense (Shamed be he who thinks ill of it) on a representation of the Garter behind the shield.
Royal.gov.uk- Coat of Arms: http://royal.gov.uk/MonarchUK/Symbols/Coatsofarms.aspx