Selectors

A tag selector (p) styles every element of that type. A class selector (.highlight) styles every element carrying class="highlight". An id selector (#header) targets the one single element with that id.

Example 1: combining selectors

p { font-family: sans-serif; }
.highlight { background: yellow; }
p.highlight { font-weight: bold; }

p.highlight (no space between them) only matches paragraphs that also carry the highlight class, combining selectors narrows what they match, while spacing them out (p .highlight) would mean something different entirely: any highlighted element inside a paragraph.

Example 2: what happens when two rules disagree

p { color: blue; }
.highlight { color: red; }
/* A 
 ends up red, class beats tag selector */

When two rules target the same element with different values for the same property, CSS uses specificity to decide the winner: id selectors beat class selectors, which beat tag selectors. This is exactly why unexpected styling sometimes "just doesn't apply", a more specific rule elsewhere is quietly winning, and learning to check specificity is often the fastest way to debug it.