Seinen manga are Japanese comics for young men. There are also seinen anime. Much the same way that shônen is meant for boys up to about high school age, seinen is meant for men older than high school age. It’s basically the next male demographic.
Seinen manga and anime have a very wide range of stories. Seinen can be almost anything that’s intended for men over 18 years of age. Action, comedy, drama and even hentai can all be called seinen.