The Twelve Disciples, or apostles, were the closest followers of Jesus and after His death and resurrection they became the primary teachers of the gospel. These twelve men were followed Jesus around during His earthly ministry and witnessed countless signs and wonders.
When Jesus was arrested and taken before the High Priest the disciples followed Him, but the only one of the twelve mentioned at the crucifixion is John. After the death of Jesus the twelve were gathered together behind locked doors when the resurrected Jesus appeared to them. He breathed on them and they received the Holy Spirit. Afterwards Jesus sent them out into the world to proclaim the gospel to all people.
Since Judas Iscariot betrayed Jesus and hung himself, the eleven apostles that were left decided to fill the vacancy. They put forward two names and cast lots for them. The lot fell on Matthias who was then added to their number.