선택자 뒤에 : 를 붙임. 

예) .header-box:hover {  }

• :link                 /* 방문한 적이 없는 링크  */
• :visited             /* 방문한 적이 있는 링크  */
• :hover              /* 마우스를 롤오버 했을 때 */
• :active              /* 마우스를 클릭했을 때     */
• :focus               /* 포커스 되었을 때 (input 태그 등) */
• :first                 /* 첫번째 요소                 */
• :last                 /* 마지막 요소                 */
• :first-child          /* 첫번째 자식                */
• :last-child          /* 마지막 자식                 */
• :nth-child(2n+1)  /* 홀수 번째 자식    */