Ko Lanta is a district of Krabi Province, consisting of a number of islands. The two largest are Ko Lanta Noi and Ko Lanta Yai. Although Ko Lanta Noi is inhabited, it is Ko Lanta Yai on which tourists will stay.
Ko Lanta is a little less well-known than nearby Ko Phi Phi but it is hardly undiscovered: the several beaches on the west coast of Ko Lanta Yai are each strung with an unbroken line of resorts and bungalows. Ko Lanta is especially popular with Swedish tourists: although the 'tourist language' is English as it is in most places in Thailand, and you will find menus and so on are translated into English, expect the poolside language to be Swedish much of the time.
Ko Lanta is popular with tourists seeking a holiday away from the parties: more walking on the beach and watching the sunset than drinking and dancing. It is also popular with families with young children, and of course, with divers.