It's actually a 'Boy's Magazine'. It covers popular things for boys aged 5-14ish in Japan including games, TCGs, anime and manga.
Coro Coro generally gets the scoop on all Pokemon stuff in their Pokemon Section as they release during the week and Nintendo of Japan's websites only update on Fridays. They get given the materials in advance, as well as a lot of exclusive interviews and can generally be relied on as THE source of all major Pokemon news.