There is no one official symbol representing the Bahá'í Faith, but there are three symbols commonly used: a nine-pointed star, the ringstone symbol, and calligraphy of the Greatest Name.
Nine Pointed Star
The most commonly used symbol is the 9 pointed star. No particular design is more desirable than others, as long as it has 9 points. The number "nine" is significant for Bahá'ís for several reasons.
Nine years after the announcement of the Báb in Shiraz, Bahá'u'lláh received the intimation of His mission in the dungeon in Teheran. Nine, as the highest single-digit number, symbolizes completeness.
The Arabic alphabet can be used to represent numbers, attaching a numerical value to words. The numerical value of Bahá is 9. The word Bahá is the root word for Bahá'í, Bahá'u'lláh, and Yá Bahá'ul 'Abhá. Bahá'u'lláh often referred to Bahá'ís in his writings as "the people of Bahá", and in addition, the Báb sent a tablet to Bahá'u'lláh with 360 derivatives of the word Bahá, fulfilling the Islamic tradition that the Promised One would reveal the "hundredth name of God". The Qur'an has 99 Names of God.
The Ringstone Symbol
Designed by `Abdu'l-Bahá , the ringstone symbol, as its name implies, is the most common symbol found on rings worn by Bahá'ís, but it is also used on necklaces, book covers, and paintings, as well as in the architecture of the Shrine of the Báb.
The ringstone consists of two stars (representing the "twin manifestations" of the Báb and Baha'u'llha) interspersed with a stylized Bahá’ (Persian for "Glory") whose shape is meant to recall the three onenesses.
The Greatest Name
The Greatest Name, or more fully, the calligraphy of the Greatest Name of God, is a calligraphized Arabic rendering of "Yá Bahá'ul 'Abhá" ("O Glory of the Most Glorious!"). It was original done by the eminent early Bahá'í calligrapher Mishkin Qalam, and later adopted by Bahá'ís everywhere.
It is generally considered to be the more reverent of the three symbols, therefore it is found in more distinguished places like paintings and art, rather than rings, t-shirts, and tattoos.