Symbols & Their Meanings...
Three is a very important number in Celtic and Wiccan traditions.  The Triquetra represents the threefold nature of the Goddess as virgin, mother and crone.  It symbolizes life, death, and rebirth and the three forces of nature:  earth, air, and water.  The inner three circles represent the female element and fertility.  The Triquetra symbol itself dates as early as the eighth century on carved stones in northern Europe.  A Norse rune known as the Odin Knot or Val Knot resembles it also.  The pagan reverence for the symbol of three was quickly picked up by the christians.  In fact, the Triquetra itself is somtimes seen on the robes of Catholic priests; as is the now widely known symbolism the christians used to entice the Irish Celts to convert, the shamrock.  Christianity re-invented the symbolism of three to mean the Father, the Son and the Holy Ghost, i.e., the Trinity.
The Shamrock, at one time called the "Seamroy", symbolises the cross and the blessed trinity.  Before the Christian era it was a sacred plant of the Druids of Ireland because its leaves formed a triad. 
Coats of Arms are part of the story of Heraldry, which began in the 11th century in Europe, and in England in the 12th century.  Arms were a symbol placed on flags, shields, and coat armour (cloak worn over armour), that provided a useful visual identification of the bearer, who was someone worthy of the Arms and needed to be recognised in battle or tournments, such as nobility and knights.  A Crest was eventually incorporated into the Arms of the bearer, and today the Coat of Arms usually refers to both the Crest and Arms, with the Crest on top of the helm (helmet) which is on top of the arms. 
A very protective and vigilant Spirit Guide, the Griffin is a master of astral workings and a guardian of the Soul and Life-essence.  It appears as a creature with the head & wings of an eagle and the body of a lion.
Yin and Yang symbolize the forces of cosmic energy and cosmic balance.  Yin is all that is "feminine" receptive and magnetic, dark.  Yang is the "masculine" active and kinetic force, light.  However as they flow together, each of these forces are shown to hold a small dot of the other within them. 
The Shield Celtic knot is a symbol for protection.  The ancient Celts used it to decorate the shields of warriors, the clothing of children, and to protect the sick.
The Pentacle is a 5 pointed star, often, but not always, encircled with the circle of Unity.  it represents the domination of Spirit or Divine Will over base matter, and the elements of Spirit, Earth, Water, Fire, and Air.  The pentacle has been a symbol of protection and spiritual growth for millennia.  At its poins are the stages of Life:  birth, initiation, consummation, repose, and death, with the circle bringing us back to birth in the cycle of reincarnation. 
Links for other symbols
Pagen, Wiccan, and New Age
Tween the Shadows My Fave - The Celtic Lady Tons of cool links on this one
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