Intro
Mission Control provides a vantage point where you have a clear view of everything that’s running on your Mac. From there, minimal effort is needed to find the window you want and bring it to the foreground, even if it’s one among dozens, saving you the hassle of moving others out of the way or pressing a keyboard shortcut to cycle through them.
Mission Control incorporates another feature, Spaces (known as virtual desktops in Windows and Linux), that further keeps things organised. Similar to how you might position tools around a physical desk, apps can be compartmentalised in whatever way makes sense to you. You might put communications apps such as Mail and FaceTime in one space, and Pages and Safari in another to take notes as you research a subject. To start using these helpful tools, first you need to know at least one way to enter Mission Control.
Step 1: Open with a trackpad

On a built-in trackpad or Magic Trackpad, place three fingers on the surface, then swipe up to enter Mission Control. If you accidentally trigger this too often, the gesture can be disabled in System Preferences > Trackpad > More Gestures. Alternatively, it can be set to require four fingers on the trackpad instead of three.
Step 2: Open with a Magic Mouse

Lightly tapping twice with two fingers on the surface of a Magic Mouse opens Mission Control. Be careful not to double-click, though. If you’re prone to doing that, try moving your hand back a bit and tapping closer to the peak of the mouse’s back. This gesture can be disabled in System Preferences > Mouse > More Gestures.
Step 3: Open with the keyboard

When your hands are on the keyboard, there’s no need to reach for another device to enter Mission Control. Press the F3 key (labeled with several rectangles in a larger one). If this doesn’t work, check in System Preferences > Keyboard that the option for function keys to work as ‘standard’ keys isn’t turned on.
Step 4: Mission Control’s layout

Mission Control consists of three areas. At the top is a row of thumbnails representing all of the Desktops, or workspaces, you’ve created. The current one’s image has a highlight around it. We’ll talk about spaces shortly. At the bottom is the Dock. The large area between them shows all of the windows that are open in the current space.
Step 5: Stacks of windows

Windows are grouped by app. Spread out a stack by rolling the pointer over it, then swipe up (one finger on a Magic Mouse, two on a trackpad). With the pointer over a window, press the spacebar for a closer look. When you’ve found the one you want, click it to return to the Desktop with it in the foreground.
Step 6: Tidy your workplace

If a space gets cluttered with too many windows, they can be spread out across multiple spaces. Start to drag a window in Mission Control and drop it onto the + that appears at the top-right corner. A new Desktop is created and the window is moved to that workspace. Click a space’s thumbnail to jump to it.
Step 7: Move between spaces

Swiping horizontally (two fingers on a Magic Mouse, three on a trackpad) moves between spaces. This works inside and outside Mission Control, so you can quickly jump between, say, Mail and Safari in adjacent spaces. With many spaces, it can be quicker to come back to Mission Control and click a space’s thumbnail to jump straight to it.
Step 8: Organise windows

Once you have multiple workspaces, windows can be moved from one to another by dragging and dropping them from the centre of Mission Control onto a thumbnail in the top row. Below an app’s stack of windows is its icon. Drag that to pick up all of its windows in one space to move them in unison.
Step 9: Assign an app

When you open an app, it normally appears in the current space. Leave Mission Control, Ctrl-click an app’s Dock icon and roll over Options. The items listed under Assign To offer the default behaviour (None), whether the app always opens in the current space (This Desktop), or make it appear in every space (All Desktops).
Step 10: Clearing Space

Up to 16 spaces can be created, which persist when empty and through restarts. If some are no longer needed, hover over one’s thumbnail until a cross appears at its top-left corner, then press it. Windows are moved to the first space. Beware that this clears that space’s assignments. Apps will revert to opening in the current space.
Step 11: Full-screen apps

The icon with two arrows at the top-right of some windows takes an app or document into full-screen mode. It appears as a dedicated space in Mission Control, though 16 more can still be manually created. Move the pointer to the screen’s top edge to reveal the menu bar, and press the far right icon to switch back.
Step 12: Rearranging spaces

Spaces can be rearranged so that apps you use in conjunction are in adjacent spaces. Click and drag a space’s thumbnail left or right to do this. OS X will automatically rearrange spaces by its own logic. If you find this overrides your personal preference, it can be disabled by unticking a setting in System Preferences > Mission Control.
Step 13: Dashboard and widgets

Dashboard is a special space at the left of all others, where widgets such as a unit converter can be pinned. If the F4 key shows a circular icon, pressing it shifts to Dashboard and back again. Otherwise, a shortcut is shown and can be changed in System Preferences > Keyboards > Keyboard Shortcuts > Mission Control.
Step 14: Alternative window arrangement

Before OS X Lion, Mission Control was preceded by a feature called Exposé, which arranged windows in a more scattershot manner, rather than stacking them up. It could be clearer with only a few apps and windows open at once, as no windows overlapped. This arrangement can be restored by unticking ‘Group windows by application’ in Mission Control’s preference pane.
Step 15: De-clutter the dock

In this view, you might see a dividing line with large window previews above, and small thumbnails, representing minimised windows, below it. Minimised windows also appear as icons in the right side of the Dock. With many minimised, Dock icons get smaller. This can be prevented by turning on ‘Minimize windows into the application icon’ in System Preferences’ Dock pane.
Bonus Tip: A clear view of one apps windows

When working in a single app, you can get a clearer view of just its windows than Mission Control provides. Leave Mission Control, move the pointer over an app’s Dock icon, then lightly tap with two fingers on a Magic Mouse, swipe down with three fingers on a trackpad, or Ctrl-click and choose Show All Windows. Only that app’s windows (from all spaces) are shown. Small window previews below a dividing line represent minimised windows and recently used files. If the Dock gets crowded with minimised windows, try turning on ‘Minimize windows into application icon’ in the Dock’s preferences.