There has always been a period of fasting before Easter in the Church, though the duration has varied quite a bit (Pohle, 1914).
Why 40 days?
The number is highly symbolic*. In the Bible, we find the Israelites wandering in the desert for 40 years. In the Gospel, we hear of Jesus spending 40 days in the desert (Mark 1:13) and, after the resurrection, 40 days with His disciples (Acts 1:3).
We do not fast on Sundays, so to make 40 days we combine 6 weeks before Easter (6 weeks x 6 days = 36 days) and half of the week prior (Ash Wednesday through Saturday = 4 days). Lent ends, then, on Holy Thursday evening.
What do we do?
The Catechism puts it this way: "By the solemn forty days of Lent the Church unites herself each year to the mystery of Jesus in the desert."  What did Jesus do in the desert? He prayed, growing in love of God the Father, and He fasted and battled temptation, growing in love of others and preparation for ministry (service).
We're to do the same two things. As Fr. John Hardon wrote, "(t)here are two guiding principles for the observance of Lent. During this season, the faithful are to grow in their love of Jesus Crucified, and they are to practice extra penance for their own and other people's sins." Again, we are to grow in love of God and love of others - which always leads back to God.
Daily reading is a great way to build your spiritual muscle - and grow in your love of God - during Lent. In this Year for Priests, it seems most appropriate to read from the patron saint of priests. Conveniently, Fr. Jerabek has put together a St. John Vianney reading plan.
If you prefer interacting with a group, Loyola Press has started their online retreat at Days of Deepening Prayer.
Finally, for parents, the Catechist's Journey blog offers 40 Ideas for 40 Days, so your children can get involved as well.
We are all called to give the very best of ourselves - not only to God but to our spouses, our families, our Church, and the world. Lent is a wonderful time to consciously make an effort to develop virtue.  There are three theological virtues, those that related *directly* to God: faith, hope, and charity (love).
Rooted in those theological virtues are the cardinal human virtues of prudence, justice, fortitude, and temperance. Fr. Robert Barron (2001) refers to these "seven lively virtues" as the counters to the classic list of seven deadly (or mortal) sins. Lent is a perfect time to start flexing those virtuous muscles in our daily lives.
* Note that the word "symbolic" does not necessarily imply fictional. For example, Jesus' 40 days in the desert can be symbolic of the Israelite exile and really have happened over a forty-day period.
Barron, Robert. "Sermon 7 : The Seven Deadly Sins and the Seven Lively Virtues." Available from http://wordonfire.org/WOF-Radio/Sermons/2001/Sermon-7---6th-Sunday-in-Ordinary-Time-----The-Sev.aspx. Internet ; accessed 10 Feb 2010.
Hardon, John. "Lent." Father John A. Hardon, S.J. Archives. Available from http://www.therealpresence.org/archives/Lent/Lent_001.htm. Internet; accessed 17 Feb 2010.
Pohle, J. "Lent." The Catholic Encyclopedia, 1914. San Diego: Catholic Answers, 2007.