ICC is the acronym of the International Color Consortium.
ICC profiles help you to get the correct color reproduction when you input images from a scanner or camera and display them on a monitor or print them. They define the relationship between the digital counts your device receives or transmits and a standard color space defined by ICC and based on a measurement system defined internationally by CIE. Thus, if you have a profile for each of your scanner, camera, display and printer, the fact that they refer to a standard color space lets you combine them so that you obtain the correct color as you get images from the scanner or camera and print or display them.
An ICC profile is one that conforms to the ICC specification. By conforming to this specification profiles may be exchanged and correctly interpreted by other users. The two main types of profiles are source (input) and destination (output) profiles and essentially consist of tables of data that relate the device co-ordinates to those of the standard color space defined by ICC. There are various relationships defined in each profile (known as rendering intents). Special types of profiles (device link, and abstract) are defined for special workflow applications.
With regard to output devices such as digital inkjet printers, an output profile simply defines the relationship between the ink and media used on a given device. However, an ICC profile cannot work on its own -- it must be matched with the correct device and settings to obtain correct results.
One additional point of confusion is created by the suffix of the filename. Apple standardized on an extension of .icc, while Microsoft standardized on an extension of .icm. Both files conform to the ICC specification and therefore the extensions are completely interchangeable.
Certain RIP software packages encapsulate their ICC profiles in proprietary containers. Onyx is one of the main culprits of this, and Onyx profiles furthermore have restrictions on distribution, meaning that you cannot simply copy profiles created in one copy of PosterShop to another unless you have paid for the distribution license.