Spacing in between class specifiers means a ascendant -> descendant relationship.

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The rule:

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.test .heading { font-weight: bold; }

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Would apply to the <p> element here:

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<span class="test"><p class="heading">Something</p></span>

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The lack of space means that the element must have both classes for the rule to apply.

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The rule:

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.heading.major { color: blue; }

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Would apply to the <p> element here:

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<p class="heading major">Major heading</p>

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.class1.class2 

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= matches element that has both classes.

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.class1 .class2

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= matches element with class2 inside element with class1.

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