What is the basic syntax of a CSS rule, and what are its parts called?

A CSS rule is a selector followed by a declaration block in curly braces, and each declaration inside is a property: value; pair.

CSS works by selecting HTML elements and assigning a style to that selection. The anatomy is small but the vocabulary matters because everyone uses these exact terms:

p {
    color: blue;
    font-size: 16px;
    margin-bottom: 10px;
}

• Selector — p here — picks which elements the rule applies to (in this case every paragraph).
• Declaration block — the { ... } — holds one or more declarations.
• Declaration — e.g. color: blue; — one styling instruction, ending in a semicolon.
• Property — color — the thing you are setting.
• Value — blue — what you set it to.

A common beginner bug is forgetting the semicolon between declarations; without it the browser may silently ignore the rest of the block. The rule shown sets every paragraph's text colour to blue, its font size to 16 pixels, and adds a 10-pixel gap below it.

Go deeper:

• CSS selectors (MDN) — the canonical overview of selector + declaration syntax and every selector type that follows.