Eid-Al-Adha is the celebration of sacrifice, and it is important for two reasons.
First, during Eid-Al-Adha we remember the life of Prophet Ibrahim (alaihi sallam) and how he was willing to sacrifice a person he loved, his son Ismael (alaihi sallam) because it was Allah's order.
Allah chose Ibrahim (alaihi sallam) to repair the Kabah from damage, and Ibrahim (alaihi sallam) got his son Ismael (alaihi sallam) to help him. They struggled for months in the hot desert, but because they wanted to please Allah they continued. After the Kabah was finished, on the 10th day of the month of Dhul' Hijja (the day we celebrate Eid-ul-Adha), Allah asked Ibrahim (alaihi sallam) to sacrifice Ismael (alaihi sallam). Amazing - Allah asked Ibrahim (alaihi sallam) to sacrifice his own son Ismael (alaihi sallam), and Ibrahim (alaihi sallam) never even questioned "why" or said "no", he trusted Allah no matter what! His faith was rewarded when Allah saved his son’s life at the last minute. Just as the knife was coming down, there was a lamb in the place of Ismael (alaihi sallam) and Ibrahim (alaihi sallam) actually sacrificed the lamb instead of his son.
Second, Eid-ul-Adha marks the end of Hajj (the 5th pillar of Islam). Every year, million's of people from all over the world travel to Mecca and perform the pilgrimage (Hajj) together. All the Muslims are united, they all wear the same clothes (sheets called ihraam), as everyone is there for the same reason, there is no competition. For a Muslim seeing the Kabah is amazing, the place on earth where we are closest to Allah. You know at home when we stand to pray, we face the direction of Qibla and in front of the Kabah you're really there! You forget about everything else in the world except what you are doing at that exact moment in time. People who are not actually joining the Hajj still celebrate Eid-Al-Adha.
Celebrating Eid-ul-Adha with your family and friends is similar to that of Eid-ul-Fitr, at the end of Ramadan. Many people sacrifice animals (like goats or lambs) for this Eid, which is a Sunnah act (something which the Prophet Muhammad did). When this is done, 1/3 of the meat is sent as charity to poor people, 1/3 is given to neighbours and friends and 1/3 you share with your family