In Usenet parlance, a crosspost is an article that's posted simultaneously to two or more newsgroups. It's considered good Netiquette to crosspost only to those groups whose subject has to do directly with your post.
In the Looniverse, crossposting is a way of traveling between the worlds contained in the different newsgroups. A net.thingee is often used for this purpose. One quick way of moving from one part of rec.arts.comics.creative to another is by crossposting through the much smaller alt.comics.lnh.