CSS follows a rule-based structure. Each rule consists of a selector and a declaration block. Selectors pick the HTML elements, while declaration blocks contain pairs of properties and values.

The general syntax for writing CSS:

selector {
    property: value;
}

Note: Semi-colon (;) at the end of each new property and property value is IMPORTANT.

For example:

h2 {
    color: blue;
}

In the example:

• h2: h2 is the selector.
• color: It's the property.
• blue: The property value.

Within the declaration block, there can be multiple pairs of properties and values.

Consider the example:

button {
    color: white;
    background-color: black;
    border: transparent;
    border-radius: 5px;
}

Here, button is the selector, and there are multiple pairs of properties and values. Each pair is separated by a semicolon ;.