Copy: Select the text or item, then press Command (⌘) + C
Paste: Place the cursor where you want it, then press Command (⌘) + V
Cut (optional): Select the text or item, then press Command (⌘) + X
Copy/Paste using menu: Select the text or item, then Edit → Copy; place cursor, then Edit → Paste
Copy/Paste using right-click: Right-click the selection → Copy; right-click where you want to paste → Paste
Copy/Paste for files/folders: Select items in Finder, then Command (⌘) + C; go to destination, then Command (⌘) + V
Move instead of duplicate (files/folders): Cut with Command (⌘) + X, then paste with Command (⌘) + V
Paste without formatting (optional): Paste and match style with Option (⌥) + Shift + Command (⌘) + V
Copy from a screenshot: Take a screenshot, then use the thumbnail to copy; paste with Command (⌘) + V