Most newsreaders allow you to post your article to more than one newsgroup. Before you post your article, you specify all of the newsgroups to which you want to post. Cross-posting is posting an article to more than one newsgroup. The newsreader simply posts the same article to each of the specified newsgroups. The article looks exactly the same as if you had posted it to each newsgroup yourself.
Most newsreaders list the cross-posted newsgroups with the article.
People replying to your cross-posted article will find their replies in each of the listed newsgroups, unless they remove some of them first. Spamming is posting an article to many or all of the newsgroups. This is similar to a mass mailing in the regular mail. Spamming often happens when someone sends out advertising for their product or service or as a marketing ploy. The article has little in common with any of the posted newsgroups.