Spam is content / techniques generated only for search engines and not humans in an attempt to inflate ranking. Intent / targeting is key. Look for? Page level - doing things useless for end-users. Link-level: in & out links. Deceiving end-users. Spammy behavior accounted for in rankings - can result in delisting.

Spam more about intent of technique and the extent a technique is used, rather than being a specific technique. Where is the fine line? Varies by industry. Is a user comfortable with the user experience of a site (excessive links, nonsensical text, etc.). Search engines can’t discose where the line is.