(Grown-up Shinichi above.)
Even though the main character - Shinichi Kudo - gets transformed into a six-year-old during the first two episodes, the stories are not necessarily for kids only. It's a crime series and so there's a crime - quite often a murder - in every episode. They are not 'nice' murders either - in the first episode a man is decapitated by a rather ingenious device while riding a roller coaster, for example. And despite the fact that the really gruesome stuff is rarely shown, it's still interesting.
('Shrunken' Conan below.)
In addition - and that's what I like most about the series - the whole episode is always created in a way to allow the audience to try and solve the crime as well ... the clues are shown - very unlike some other series.
(Ran and Kogoro Mori above.)
Shinichi is 'shrunken' by two mysterious 'Men in Black' (not Will Smith and Tommy Lee Jones) and then moves in with his friend Ran and her father, a rather unsuccessful private investigator to find a trace of them. Unfortunately - as the originally 16-year-old now looks like a boy of six - he's forced to go back to primary school where he meets new three new friends.
(Conan's new friends below.)
His only help comes from his former neighbour (Shinichi's parents, a former actress and a writer of crime-stories, live abroad), a scientists who develops various gimmicks for him, like a bow-tie that allows him to imitate various voices, sneakers that allow him to kick a foot-ball (or other object) strong enough to bring an adversary down and a tracker that can be placed on any surface and can be followed with a little transparent monitor build into Conan's fake glasses. Those glasses, together with a little microphone also allow him to listen into conversations.
(Conan's neighbour below.)
The creator of the series, Gosho Aoyama, has chosen the names of most of his main characters carefully. "Conan Edogawa" is a combination of the two crime-story authors Arthur Conan Doyle (creator of Sherlock Holmes) and Rampo Edogawa (a very famous Japanese writer). Kogoro Mori has the same first name as Rampo Edogawa's detective Kogoro Akechi. The police inspector called into most cases is named Inspector Megure, a name which sounds a lot like "Maigret", George Simenon's famous inspector. Finally Conan's neighbour, the professor, is named Agasa, which sounds almost like "Agatha", the first name of Agatha Christie.
The series is still running - both as a manga and as an anime - and Conan still is a little boy, but by now a lot of different characters have appeared, among them a rival school-boy detective, a girl 'shrunken' by the same poison (she used to work for the 'Men in Black') and a mysterious thief - who actually comes from another manga the inventor of Conan has created: Kaito Kid ('kaito' is the Japanese word for 'thief').