This article is taken from one that contrasted the terms for both "good" and "evil." This article only covers the terms translated as "good." The original larger article is here.

There are three different Greek words commonly translated as "good" in Christ's words, kalos, agathos, and chrestos. There is also a prefix, eu, that is used in many words that are translated as "good." For example, the idea of the gospel as "good news" comes from this prefix in euaggelio.

None of these words means "good" in quite the same sense as our English word. All are more specific in the quality that they describe. The fact that biblical translation conflates all three ideas when Jesus saw them as separate makes it difficult to understand what Jesus is saying in any given verse.

In some verse, Jesus contrasts two different Greek words that both get translated as "good." This completely misses his point. For example, many English translations of Matthew 7:17 described both the tree and its fruit as "good." Jesus, however, used two different Greek, saying very specific things about the nature of the tree and the nature of its fruit.  He could have used the same adjective to describe both the tree and its fruit, but he chose not to. Why? The possible reasons are lost in lazy translation.

The Good of Kalos

The most common Greek word translated as "good" is kalos. This word means “beautiful,” “of fine quality,” “noble,” and “virtuous.” This word appears about three times more often than the other word, agathos, which is the second most common Greek word translated as "good."

This word is used in the many verses to describe both good acts and good things (Matthew 3:10 Matthew 7:17“good fruit,” Matthew 5:16 “good works,” Matthew 5:44 “do good,” Matthew 12:33 “good tree,” Matthew 13:8 “good ground,” Matthew 13:24, “good seed,” Matthew 17:4 “good for us,” Matthew 26:24 “good for that man”).

The sense of kalos is possible best captured by the concept of "fine quality." When a something is kalos, it is praiseworthy and easy to appreciate.

The Good of Agathos

The other Greek word translated as "good" is agathos. Agathos, when applied to things, means “good” in the sense of “sound,” “serviceable,”“useful,”beneficial," and “correct.” When applied to people, it primarily “well-born,” “gentle,” “brave,” “capable,” and "correct.” Agathos is not used to describe good things except to refer to good deeds (as is kalos ) and good people (’the good”).

Agathos is closer to our concept of "correct" and "useful." Jesus often contrasts agathos with poneros. )See this article on "evil." This contrast is usually translated as some form of "good and evil." Throughout the Gospels, agathos is translated primarily as "good" in the sense of virtuous. However, this is misleading as well. The contrast here is less between good and evil, but between useful and worthless, well-born and oppressed, healthy and second-rate.

When used as a noun, agathos is usually translated as "goods" in the NT, but in English there is no real connection between the moral concept of "good" and the property of "goods". Perhaps a better translation would be "valuables" to get us away from the whole "good" and "evil" dichotomy. The real contrast is between the valuable and the worthless. 

Agathos is from the same roots as agape and agapao, the Greek words usually translated as "love," but whose real meaning is explained in more detail here.

The Good of Chrestos

Another word that is that can be translated as "good" is chrestos (χρηστός ) but the word is used only twice. It refers to "good" wine in  Luke 5:39, "kind" acts in Luke 6:35, and "easy" in Matthew 11:30.

This word means "good", "useful", "good of its kind," and "serviceable;" of persons, "good", "kindly;" "honest", "worthy," in war, "valiant", "true;" of the gods, "propitious", "merciful", "bestowing health or wealth;" of a man, "strong", "able in body for sexual intercourse;" when used as a now, "benefits", "kindnesses", "happy event", "prosperity," and "success."

In a moral sense, chrestos is also the opposite of kakos, which means "bad" and "evil," but which is not the word usually translated as "evil" in the NT. For more on this topic, see this article