This is the one that catches everyone: ⌘Tab moves between apps. Two Word documents are one icon there, and pressing it lands you on whichever window was last in front.
- ⌘` (backtick, the key above Tab on a US keyboard) cycles through the windows of the app you are in. Add ⇧ to go backwards.
- The app's Window menu lists every open window at the bottom; a tick marks the current one, a diamond a minimized one.
- ⌃↓ (or a three-finger swipe down) shows every window of the current app spread out; click the one you want.
- Click and hold the app's Dock icon, or right-click it, for the same list.
A minimized window does not come back with ⌘Tab; it stays in the Dock until you click it or pick it from the Window menu. Windows on another desktop switch you to that desktop when chosen.