Advent is the Preparation, in expectancy of the Messiah. It is a season or time of preparation that directs our hearts and minds to Christ’s second coming at the end of time and also to the anniversary of the Lord’s birth on Christmas. Lent is Penance.
According to The Catechism of the Catholic Church (524), Advent is: When the Church celebrates the liturgy of Advent each year, she makes present this ancient expectancy of the Messiah, for by sharing in the long preparation for the Savior’s first coming, the faithful renew their ardent desire for his second coming. In preparation, we are encouraged to “humble ourselves and become little.”
Lent, on the other hand, according to the Catechism (1438): Is the seasons and days of penance in the course of the liturgical year (Lent, and each Friday in memory of the death of the Lord) are intense moments of the Church’s penitential practice. These times are particularly appropriate for spiritual exercises, penitential liturgies and pilgrimages as signs of penance, voluntary self-denial such as fasting and almsgiving, and fraternal sharing (charitable and missionary works).
And (540): By the solemn forty days of Lent, the Church unites herself each year to the mystery of Jesus in the desert.
And according to The USCCB: Lent has had a different history than Advent among us. Beginning with the powerful lesson of Ash Wednesday, it has retained its ancient appeal to the penitential spirit of our people. It has also acquired elements of popular piety which we bishops would wish to encourage. Accordingly, while appealing for greater development of the understanding of the Lenten liturgy, like that of Advent, we hope that the observance of Lent as the principal season of penance in the Christian year will be intensified.
Therefore, it is Clear that while both seasons have a character of both preparation and penance, Advent is mainly preparation, and Lent is mainly penance.
Advent is like the building up, a solid foundation upon which the Christmas season is set up. while Lent is like tearing down and clearing away of brush and sticks and old dwellings, so Easter can stand.
This is mirrored in our decorations during advent. In Advent, we slowly build up adding elements in, one or two at a time. For our nativity set, we set up the stable first, and add the animals and shepherds, then Mary and Joseph, then the baby Jesus and the angel on Christmas, and finally the wise men on Epiphany. All those decorations that went up during Advent remain for the entire Christmas season. (At least through Epiphany, and sometimes through the Baptism of the Lord or maybe even until Candlemas.)
Lent has a different feeling than Easter. During Lent, we place on our tables or on our altars, purple cloths, some others use rocks, very big nails from the hardware store, a crown of thorns for decorations. This decoration changes on the 5th Sunday of Lent, where we veil all the crucifix in our homes. Many churches veil all statues and religious images.
Note that both seasons involves waiting and are other differences, of course, between Lent and Advent, notably fasting and voluntary Lenten disciplines.