What is Lent?

The Ash Wednesday Mass has an introduction that says:-

Brothers and sisters in Christ: since early days Christians have observed with great devotion the time of our Lord's passion and resurrection. It became the custom of the Church to prepare for this by a season of penitence and fasting.

At first this season of Lent was observed by those who were preparing for Baptism at Easter and by those who were to be restored to the Church's fellowship from which they had been separated through sin. In course of time the Church came to recognize that, by a careful keeping of these days, all Christians might take to heart the call to repentance and the assurance of forgiveness proclaimed in the gospel, and so grow in faith and in devotion to our Lord.

I invite you, therefore, in the name of the Church, to the observance of a holy Lent, by self-examination and repentance; by prayer, fasting, and self-denial; and by reading and meditating on God's holy word.

The Church of England Web site says:-

We traditionally think of Shrove Tuesday as a day for 'using up' ingredients in the house to remember how Christians may choose to abstain from various types of food during Lent - but there is a lot more to Lent than making pancakes.

Lent starts on Ash Wednesday and ends on Easter Eve in the Western Churches.
Many Christians focus with a fast on Ash Wednesday and again on Good Friday.
Some will fast for the time between Ash Wednesday and up to Easter, others don't fast on Sundays as Sunday can be considered to be a feast day.
There are various ways of fasting, some people will choose to give up bad habits or certain foods and drinks or meals, where as others prefer to see Lent as a way to help others, pray for others, focus on praying for others or take up something new. There are many Christians who attempt to do both.
The concept of fasting for 40 days is to reflect the 40 days Jesus' fasted in the desert. Christians treat this time as a way of preparation to Easter and fasting is a recognised way of helping people to focus on their spiritual life and prayer.
There are other Biblical references to the importance of the number 40. The flood in the book of Genesis was 40 days. The Hebrews were in the wilderness for 40 years before reaching the promised land. Moses fasted for 40 days prior to receiving the 10 Commandments.

Times and Seasons (a liturgical resource book) says of Lent:-

Lent may originally have followed Epiphany, just as Jesus’ sojourn in the wilderness followed immediately on his baptism, but it soon became ﬁrmly attached to Easter, as the principal occasion for baptism and for the reconciliation of those who had been excluded from the Church’s fellowship for apostasy or serious faults.This history explains the characteristic notes of Lent – self-examination, penitence, self-denial, study, and preparation forEaster, to which almsgiving has traditionally been added.Now is the healing time decreed for sins of heart and word and deed, when we in humble fear record the wrong that we have done the Lord. (Latin, before 12th century) As the candidates for baptism were instructed in Christian faith, and as penitents prepared themselves, through fasting and penance, to be readmitted to communion, the whole Christian community was invited to join them in the process of study and repentance, the extension of which over forty days would remind them of the forty days that Jesus spent in the wilderness, being tested by Satan. Ashes are an ancient sign of penitence; from the middle ages it became the custom to begin Lent by being marked in ash with the sign of the cross.The calculation of the forty days has varied considerably in Christian history. It is now usual in the West to count them continuously to the end of Holy Week (not including Sundays), so beginning Lent on the sixth Wednesday before Easter, Ash Wednesday. Liturgical dress is the simplest possible. Churches are kept bare of ﬂowers and decoration.Gloria in excelsis is not used.The Fourth Sunday of Lent (Laetare or Refreshment Sunday) was allowed as a day of relief from the rigour of Lent, and the Feast of the Annunciation almost always falls in Lent; these breaks from austerity are the background to the modern observance of Mothering Sunday on the Fourth Sunday of Lent. As Holy Week approaches, the atmosphere of the season darkens; the readings begin to anticipate the story of Christ’s suffering and death, and the reading of the Passion Narrative gave to the Fifth Sunday its name of Passion Sunday.There are many devotional exercises which may be used in Lent and Holy Week outside the set liturgy.The Stations of the Cross, made popular in the West by the Franciscans after they were granted custody of the Christian sites in the Holy Land, are the best known.