We fast in order to imitate Our Lord's forty day fast in the desert. Unlike Him, we need to tame the concupiscence of the flesh, acquire self-discipline, and atone for our personal sins. It was by breaking God's commandment to abstain from eating a certain fruit that Adam and Eve lost the earthly paradise. Both Moses and Elias fasted for forty days before encountering the living God. (Exodus 24:18 and 3 Kings 19:8)
In former times, every day of Lent (except for Sundays and first-class feasts) was a fast day. Now, only two fast days remain -- Ash Wednesday and Good Friday. Ash Wednesday and Good Friday are also days of abstinence from meat, as are all Fridays of Lent. Many people think that the Second Vatican Council did away with Friday abstinence, but it did not. Every Friday of the year is a day of abstinence from meat unless the bishops of the country decide to substitute another form of penance. In the United States, it is up to every individual to perform some other Friday penance if they are not able to abstain from meat. However, every Friday of Lent remains a day of strict abstinence.
The third Lenten good work is almsgiving, by which we overcome the "world," that is, the love of riches, luxuries, and honors. Through almsgiving we not only help the poor, the missions, and the temporal needs of the Church, but we mortify any any inordinate desires for material things. By having Masses offered, our alms can assist the "Church Suffering" in purgatory. As the aged Tobias said to his son: "Prayer is good with fasting and alms more than more than to lay up treasures of gold. For alms delivereth from death, and...purgeth away sins." (Tobias 12:8-9) As Jesus commands in the Gospel for Ash Wednesday: "Let not your right hand know what your left hand is doing." (Matthew 6:3) It is most important that all our good works are accompanied by charity, humility, and the desire to please God alone.