Social engineering is the art of swindling. Tricking someone into giving you information, access into a building, a physical object, or money. Theres a few ways of going about doing this
Phishing is spam that pretends to be a reputable source. Have you ever received an email from Amazon, but it looks suspiciously amateurish, and claims that if you click a link in the email you will receive a free gift card? That's the bait. The phisher wants you to click that link which surely will not lead to amazon. There are also vishing (voice phishing) and smishing (sms phishing) that can occur
Impersonation is right what it sounds like; that attacker puts on a character to try and gain your trust. This can be as simple as an attacker calling your office phone, claiming to be "IT" and that they "really need your username and password for a backend update", or an attacker pretending to be a contractor who shows up one day "to fix the cabling in the plenum".
Dumpster diving took me by surprise when I first learned of it, but it does make sense as a potential attack vector. There's a lot of information that an attacker could learn just by digging through someones discarded mail. Most folks I know don't shred their mail or other documents before disposing of it. It's free personal info!