"Characteristics" and "qualities" are close enough in meaning that they're often used interchangeably, and in a lot of everyday sentences, swapping one for the other changes almost nothing. But they do carry a genuine difference in emphasis, and knowing it helps you pick the more precise word when it matters.
Definitions
A characteristic is a distinguishing feature of something, a fact about what it is or how it behaves, described neutrally. A quality is also a distinguishing feature, but the word carries a stronger sense of degree or value: qualities are often good or bad, strong or weak, in a way characteristics don't automatically imply.
The key difference: neutrality vs judgement
"Being tall" is a characteristic, a neutral fact. "Being generous" reads more naturally as a quality, because it already carries a positive judgement built into the word itself. This is the clearest practical distinction: characteristics describe what something is, qualities more often describe how good or valuable it is at being that.
Where the words overlap
In practice, this line blurs constantly. "Patience" can be called either a characteristic or a quality of a good teacher without sounding wrong either way. Context, not a strict rule, usually decides which word a native speaker reaches for.
Examples in context
"The characteristics of the material include its density, melting point, and conductivity" reads naturally because these are neutral, measurable facts. "Her best qualities are her patience and honesty" reads naturally too, because these are being praised, not just described. Swap the words and both sentences still make sense, but they lose a little of their natural fit.
When each word is normally used
Characteristics is generally the better choice in scientific, technical, or classification contexts, describing what defines a category, whether that's a species, a system, or a concept. Qualities is generally the better choice when evaluating something, describing a person's virtues, a product's strengths, or what makes something desirable.
A quick comparison
Characteristics: neutral, descriptive, used for classification and identification, doesn't inherently imply good or bad. Qualities: often evaluative, used to praise or assess, frequently implies a positive judgement unless specified otherwise (as in "negative qualities").
The takeaway
Both words describe distinguishing features, but characteristics leans neutral and descriptive while qualities leans evaluative. Context does most of the real work in deciding which one fits a given sentence, and native speakers move between them constantly without any strict rule forcing the choice.