The period of Lent is the 40 days that lead up to Easter (not counting Sundays). The first day of Lent is Ash Wednesday. The 40 days is significant, as it commemorates the 40 days Jesus fasted in the desert and the 40 years the Israelites spent roaming in the desert prior to entering the Holy Land. Since Sundays are a time of celebration representing the resurrection in Christianity, they are not included in the 40 days.
During Lent, many Christians will slow down and focus on their relationship God. In order to prepare for Easter, they will observe a period of fasting, repentance, moderation and spiritual discipline. Lent is about refocusing on God, and observing a time of sacrifice to symbolize the ultimate sacrifice that Christ gave on the Cross. Lent ends at Easter when, as Christians, we celebrate Christ's resurrection.