The Great British Royal Coat of Arms

The main element of the Royal Arms is the shield which is divided into four quarters (see diagram). The three golden lions on a red background, symbolising England, occupy the first and fourth quarters. The Arms of Scotland, a red lion rearing on its hind legs inside a red border, are in the second quarter, and the Arms of Ireland's golden harp with silver strings on a blue background - are in the third quarter.
The lion and the unicorn supporting the shield represent England and Scotland respectively. They stand on a small frame called the compartment which sometimes incorporates the plant emblems of Scotland (thistle), Ireland (shamrock) and England (rose). The Compartment also has the sovereign's motto usually Dieu et mon droit ("God and my right").
Around the shield is a belt or strap with the motto Honi soit qui mal y pense ("Evil to him who evil thinks"), the symbol of the Order of the Garter. Above the shield is the gold barred helmet with the crest, a royal crown topped with a golden crowned lion.

The Royal Arms we see today have evolved over nine centuries, since Richard 1st (the Lionheart) chose a shield of three lions to represent the three areas England, Normandy and Aquitaine - which were associated with the English crown. His Mother was Eleanor of Aquitaine (wife of Henry II) and his great great gandfather was William 1st (the Conqueror) Duke of Normandy.
This symbol on the King's shield would immediately identify him in the midst of battle. Other elements of the Arms also have their origin in mediaeval armour: the shield, the helm (helmet) on top of the shield, the crest above the helmet (which would also identify the King in battle) and the mantling, based on the small protective cape hanging from the helmet.
The Royal Arms were also used to symbolise the King's authority and, after Henry VIII became Supreme Head of the Church of England in 1534, they began to appear in churches, representing the connection between the monarch and the church.
The accession of King James VI of Scotland to the throne of England in 1603 brought together the Royal Arms of Scotland, Ireland and England, still the main elements of the Royal Arms today However, until 1801, the Royal Arms also contained the Arms of France (three gold fleurs-de-lis on a blue background) in one quarter, dating from the claim to the French throne made by Edward III in 1340.
The Arms of the Stuart kings remained very similar to those of James 1st. The first main change occurred in the reign of William and Mary (1688-1702) when William's Arms (a golden lion on a blue background) were placed on a small shield in the centre of the main shield (known as a shield of pretence).
The Union with Scotland in 1707 was marked by placing the Arms of England and those of Scotland side by side in the first and fourth quarters. The fleurs-de-lis of France took over the whole second quarter, and Ireland's harp the third.
The succession of George, Elector of Hanover, to the throne as George 1st in 1714 led to further changes to include his territories. The English and Scottish lions remained side by side in the first quarter, but were replaced by a completely new coat of Arms divided into three, in the fourth quarter, showing two golden lions on a red field, a blue lion on a gold background and in the lowest part a galloping white horse on a red background, the symbol of Hanover. This is often the easiest feature to identify from a distance.
The Act of Union with Ireland in 1801 caused a further reshuffle, with the Arms of England, Scotland and Ireland placed in the quarters where we see them today and the dropping of the French Arms. The Hanoverian white horse was moved to a small shield of pretence, with the Elector's bonnet on top, replaced with a crown when Hanover became a kingdom in 1816.
The final significant change occurred when Victoria became Queen in 1837. As a woman she could not succeed to the throne of Hanover, so the shield of pretence was omitted from her Arms Apart from changes in the initials of the Sovereign, the Royal Arms have remained the same ever since.

Scottish Royal Coat of Arms
Scotland uses a different version of the coat of arms which shows the unicorn on the Dexter (right-hand) side holding the Scottish flag, and the lion on the Sinister side holding the English flag symbolising the Union of the Crowns. The lion of Scotland is in the first and fourth quarters of the shield, and the red lion atop with the motto 'In Defens', which is a contraction of the motto 'In My Defens God Me Defend'. The motto of the Order of the Thistle 'Nemo me impune lacessit' appears on a blue scroll overlying the compartment. It is often translated as 'No one attacks me with impunity', or rendered in Scots as 'Wha daur meddle wi' me?'

Welsh Royal Coat of Arms

The Tudor Royal coat of arms included the red Welsh dragon which was replaced by the Scottish unicorn when the crowns of England and Scotland were united in 1603. Today the Royal coat of arms of the Prince of Wales and the Royal Badge of Wales incorporate the four lions passant of Prince Llywelyn of the dynasty of Gwynedd, which was he last independent ruling house of Wales before it was incorporated with England in 1284. The badge is encircled by a wreath alternating of leek, thistle, clover, leek and rose. The motto is 'Pleidiol Wyf I'm Gwlad' which is Welsh for 'I am true to my country'.