Classes are applied to elements to specify how you want them designed. They can be used multiple times on a page. In CSS classes are used with a .before the identifying name.
Id's are also used to specify how and element is designed, but can only be used once. In CSS id's are used with a #before the identifiying name.
CASCADE - styles can fall (cascade) from one style sheet to another, enabling mutliple style sheets to bused in one html document.
INHERITANCE - the mechanism through which certain properties are passed from parent elements down to it's children.
SPECIFICITY - The browser decides which css property values are most relevant to an element and applied - applying to something specific.