A quorum of 10 Jewish men above the age of Barmitzvah.
Some key prayers within the standard services can only be said with a Minyan, including
It is therefore best to pray with a Minyan. There is also the concept that if you aren't perfect (and let's be honest, who is!) then G-D may see this in you if you're praying alone. But if you're praying with others, then it's harder to see each person's individual bad points.
The word "Minyan" is also used to indicate the organisation of a group specifically for prayer. For example, someone who has lost a parent says the Kaddish prayer every day for 11 months. You might "try to organise a Minyan so he can say Kaddish".
Most non-Orthodox branches of Judaism will include women above the age of Batmitzvah in a Minyan.