Use the adjective detailed when you're talking about something that's full of specific points or facts. A detailed weather report includes temperature, cloud cover, and the possibility of rain.
Something that's detailed is focused on the fine points or small aspects — a detailed portrait of your grandmother includes every wisp of hair and subtle shade of skin. Detailed explanations take some time, because the person doing the explaining isn't leaving any detail out. The literal Old French meaning of the noun detail is "a cutting in pieces," and it's used to mean "small piece or quantity."