Modifier keys for Mac systems are the Shift, Control, Option (Alt), Command, and Caps Lock keys. The commands triggered by combinations of these keys can greatly speed up how you use your computer, but it can slow you down again if you switch to a keyboard with an unfamiliar layout.
While most Mac keyboards have these keys in the same relative locations, so you don't have to change your typing habits, there are situations in which you may end up using something different, such as if you take a laptop to the office and plug in whatever keyboard you happen to find. For example, on PC keyboards the equivalents of the Option and Command keys (the Alt and Windows keys, respectively) are often swapped in location.
Granted, you can adapt to the changes, or make sure you use the same keyboard type everywhere you go, but an easy alternative in OS X is to customize the system's modifier keys.
To do this, open the Keyboard system preferences and click the button called "Modifier Keys..." at the lower right. A small window will pop up in which you can select the keyboard attached to your system and assign OS X modifier functions to keys the keyboard's layout. You may only need to swap the mapped Option and Command keys, but you can set others or disable a modifier key if desired.
Keep in mind that these settings are device-specific, so you'll need to set them again for each new keyboard.