Spam means different black hat ways to manipulate search engine indexes. There are a few types of spam in SEO:
Content spam: keyword stuffing, hidden or invisible text, meta-tag stuffing and other manipulations, doorway pages, scraper sites, article spinning, machine translation.
Link spam: link farms, private blog networks, hidden links, spam blogs and guest blog spam, comment spam, wiki spam, cookie stuffing and much more.
Other types: mirror websites, URL redirection, cloaking.
Keep in mind that these spam SEO techniques will not give you long-lasting results.