In general the short and the full from are interchangeable with slight variation in meaning. Look at the example: Она счастлива. -- She is happy right now. Она счастливая. -- She is happy in general. Another one: Ребёнок болен. -- It means the kid gets ill and doesn't feel good right now. Ребёнок больной. -- It means the kid has a weak health. The full form of the adjective could be seen more often in spoken Russian. It is usually used in both meanings, although this is not perfectly correct.

Краткое прилагательное - только сказуемое в предложении. Полное прилагательное - определение или часть сказуемого. Могут имет различные значения она хороша- Beautiful, charming она хорошая- Properly brought up, kind sometimes short adjectives have no full form: Горазд, рад, надобен и др. "Служить бы рад, прислуживаться тошно." "Ну, братец, ты и спать горазд!"