What are venial sins and mortal sins (with regard to the Catholic Church)?

In regard to Roman Catholicism, a mortal sin is one that directly contradicts a soul's connection with the grace of God and therefore severs it, meaning that a soul that has committed a mortal sin without atoning for it is doomed. A venial sin is a less serious crime and can often occur without the sinner's knowledge.
Despite popular belief, the difference in a mortal sin and a venial sin can be different variations of the same wrongdoing. For example, if one gossips idly among friends about another person, this could be considered a venial sin. However, if one gossips about another person with intent to destroy and defame their reputation, the transgression would be considered a mortal sin. In regard to Inferno, all of the canonical souls in Dante's Hell are guilty of mortal sin, with the exception of the virtuous non-believers in Limbo. Even Francesca and Paolo, who were given poor circumstances and had good reasons for doing what they did, were fully aware of the gravity of their sin and, therefore, are doomed to the second circle of Hell. They are, however, offered some solace by being given each other's company.

In Catholic doctrine, there are two types of sins a person can commit: venial sins and mortal sins. Venial sins are relatively minor, such as being impatient, gossiping, or stealing something of low value. Venial sins are thought to harm an individual’s relationship with God but not irreparably. Mortal sins, however, are those that so egregious that they kill one’s spirit, destroying feelings of charity and grace within him, and thus separating the individual from God. Mortal sins include murder, fraud, and adultery. One may commit a venial sin accidentally, without malice of forethought, but mortal sins are committed deliberately, with a person’s full knowledge.
Dante’s Circles of Hell are modeled on the work of Sir Thomas Aquinas, who, some fifty years prior to Dante’s writing, ranked sins according to their level of offense against God, from least to greatest. Aquinas’s “system” has influenced philosophers, theologians, and writers for hundreds of years.
The worst of the worst sins, according to Aquinas, are known as the “Seven Deadly Sins.” They are:

Lust


Gluttony


Avarice (Greed)


Sloth (Laziness)


Anger


Envy


Pride


Additionally, all sin can be categorized as one or more of the following types, from least to greatest offense:

Level One: Incontinence, or lack of self-control
Level Two: Violence, a deliberate violation of God’s will
Level Three: Fraudulence or Traitorousness, using one’s intellect as a weapon