Queen Mab was the earliest known ruler of the Third Race, who were known in that period as the Children of Mab.  She was the mother of Oberon by an unknown father. It has also been suggested that she may have had more than one husband, although this is not certain.  Mab was apparently a tyrannical ruler.
During Mab's reign, Oberon began a relationship with Titania. Mab disapproved of the relationship, considering Titania to be "a spoiled brat and beneath her son".  Eventually, Oberon overthrew Mab in a great war at some time after the birth of Merlin in the 5th century.  With some help (and after several others cast their lot between the son and mother), Oberon defeated Mab and became Lord of the Fair Folk, who became known as the Children of Oberon.  Mab herself was imprisoned, although it is possible that she will escape in the future. 
Queen Mab is the most powerful of the Third Race, even stronger in magic than her son Oberon.  She is also reportedly even more arrogant than he is. Greg Weisman has said that she was "Pretty much born crazy".  Her favored physical form is uncertain, although Greg has mentioned that she looks basically humanoid, but with more than one pair of arms, and is quite small in stature (approximately three inches tall).  She is also connected to power and chaos. 
Real World Background
Queen Mab was the original "faerie queen" in English legend and literature, appearing in that role in such works as Michael Drayton's Nimphidia (in which she was also depicted as being married to Oberon); in Elizabethan and Jacobean times, indeed, she held that title far more often in literature than did Titania. Her best-known literary appearance is in William Shakespeare's Romeo and Juliet, where she is described by Mercutio in a famous monologue as the "fairies' midwife", who is responsible for dreams, and is of tiny size. Some scholars of faerie mythology have equated her with Queen Maeve of Connacht, the nemesis of Cu Chullain in Irish legend, although not everyone agrees with this. Queen Mab still appears in works of fantasy even today, her most recent appearance of note being in the NBC mini-series Merlin (1998), where she is the Queen of the Old Ways, and first the creator, then the bitter enemy, of Merlin.
- Mab at Wikipedia, the Free Encyclopedia