Mac keyboards have Option and Command keys where standard PC keyboards have Alt and Windows keys. The keyboards are otherwise pretty similar, but new Mac users will need to understand these different keys.
Apple's Mac keyboards actually do have a Control (Ctrl) key, but the Control key doesn't function like the Control key on Windows. Keyboard shortcuts like Ctrl+C to copy text won't work.
The Command Key
The Command key doesn't do anything on its own. It's a modifier key you can press to issue keyboard shortcuts to applications. For example, while you press Ctrl+C, Ctrl+X, and Ctrl+V to copy, cut, and paste on Windows, you press Command+C, Command+X, and Command+V to do the same on a Mac.
This key has the ⌘ symbol on it. This symbol appears throughout the Mac's menus to indicate when you can press the Command key along with another key to issue a keyboard shortcut. The Command key originally had an Apple logo on it, but Steve Jobs thought displaying the Apple logo throughout the original Macintosh's menu would be overusing the logo. A designer chose the ⌘ symbol to replace it. It's an old symbol used in Nordic countries to indicate places of interest -- in Sweden, it's the official sign for a tourist attraction.
In summary, on a Mac, you'll probably be pressing the Command key to issue keyboard shortcuts. The Control (Ctrl) key is also present, but it isn't used for as many things.
The Option Key
The Option key functions similarly to the AltGr key on many PC keyboards, which explains why it also has "Alt" printed on it. Holding it and pressing another key allows you to type a special character that doesn't normally appear on the keyboard. For example, pressing Option+4 with the US keyboard layout will produce ¢, the cent sign which normally doesn't appear on your keyboard. Like other modifier keys, it's also used as part of some keyboard shortcuts.
This key has the ⌥ symbol on it. This symbol is used throughout the Mac's menus to indicate when you can press the Option key, the same way the Command key's symbol is. Unlike for the Command key, we don't have any history that indicates why this symbol was chosen.
For example, when you click the Apple menu you'll see the keyboard shortcut assigned to Force Quit. if you're not used to a Mac's keyboard, these symbols may look like hieroglyphics -- but they're really saying you should press Option+Command+Escape to open the dialog where you can force-quit applications. It's like the Task Manager on Windows.
Modifier Key Symbols
To perform the keyboard shortcuts shown throughout Mac OS X, you'll need to know the symbols. Aside from ⌘ representing Command and ⌥ representing Option, ^ represents Control while ⇧ represents the Shift key.
If you'd like to change which key does what, you can customize this by clicking the Apple menu, opening System Preferences, selecting the Keyboard icon, and clicking Modifier Keys. You can also set the Caps Lock key to "No Action" here, effectively disabling the Caps Lock key on your Mac.
Option and Command Keys in Windows
When running Windows on your Mac via Boot Camp, the keyboard mappings are changed so they make more sense in Windows. The Option key functions as Alt and the Command key functions as the Windows key.
This can be a bit confusing when going back and forth between OS X and Windows. For example, you'll have to press Command+C to copy text in OS X, but you'll have to press Ctrl+C to copy text in Windows. These keys are in different places, so it can interfere with your muscle memory. To solve this problem, you can use SharpKeys to remap the Command and Ctrl keys in Windows.
In Mac OS X, you could also use the Modifier keys dialog to swap the functions of the Ctrl and Command keys, if you like. This would make your Mac's keyboard shortcuts work more like the keyboard shortcuts on a Windows PC.
The Command and Option keys may seem a bit foreign, but everything works fairly similarly on a Mac. The ⌘ and ⌥ symbols are printed on the keyboard so you can more easily understand the keyboard shortcuts shown throughout Mac OS X.
Image Credit: Wesley Fryer on Flickr