The most authoritative answers can be found in Article 8: Sin of the Catechism of the Catholic Church. It is carefully worded and addresses the questions asked. The following is an attempt at a supplement. Other sources include the Catholic Encyclopedia and the Baltimore Catechism.
This is about actual sin (as in "act-ual", from Latin actus) having to do with acts, and not Original Sin. Acts include willful deeds, failures to act (omissions), utterances (speech), desires, and thoughts.
The major distinction is between venial and mortal sins: something very serious accompanies every mortal sin, so mortal sins are set apart.
Sin is any act contrary to the eternal law. A sin is a willful act contrary to reason and the moral law inscribed into the conscience of every person.
The object of a sin has to do with the objective (object-ive) elements of the sin, circumstances, etc.
Grave matter is specified by the Ten Commandments. Grave sin is sin whose object is grave matter
Mortal sin is sin whose object is grave matter (a grave sin), and which is committed with full knowledge (on the part of the intellect) and deliberate consent (on the part of the will).
1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God's law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart do not diminish, but rather increase, the voluntary character of a sin.
Venial sin is any sin that is not a mortal sin:
1862 One commits venial sin when, in a less serious matter, he does not observe the standard prescribed by the moral law, or when he disobeys the moral law in a grave matter, but without full knowledge or without complete consent.
The reason we talk about grave sins is that we cannot know if another person truly committed a moral sin without knowing if they acted with knowledge and consent. But we can know on the basis of certain objective qualities that it was a grave sin, so we are able to call it that. There isn't always a clear distinction between grave and light matter. Objectively, some sins "admit of no lightness of matter", such as blasphemy or hatred of God. Other sins admit lightness. For example, intentional theft is sometimes only venial, such as when one wilfully steals paperclips from the office. It is especially difficult for a person to self-evaluate the gravity of one's own sins, which is why the Church trains priests to recognize the gravity of sins during Confession.
┌─ ─ ─ ─ ┴ ─ ─ ─ ─┐
│ venial │
┤ ╔ ═ ═ ═ ═╗
Knowledge │ venial ║ Mortal ║ ⬅"deliberate
& Consent └─ ─ ─ ──╚═ ═ ═ ═ ╝ sins"
In other words:
knowledge and consent + grave matter = Mortal Sin
From the Catechism of the Catholic Church:
Mortal sin destroys charity in the heart of man by a grave violation of God's law; it turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to him. [... Mortal sin] necessitates a new initiative of God's mercy and a conversion of heart which is normally accomplished within the setting of the sacrament of reconciliation. [...] Unrepented, it brings eternal death.
Venial sin allows charity to subsist, even though it offends and wounds it. [...] Venial sin constitutes a moral disorder that is reparable by charity, which it allows to subsist in us.
From the Baltimore Catechism:
This sin is called mortal because it deprives us of spiritual life, which is sanctifying grace, and brings everlasting death and damnation on the soul.
The effects of venial sin are the lessening of the love of God in our heart, the making us less worthy of His help, and the weakening of the power to resist mortal sin.
There are also capital sins (vices), material and formal sins, and internal sins. For example, mortal or venial sins might be internal. But all that's another question. The major distinction is between mortal and venial sins.