Although the fifth-generation Intel Core i5 processor and Force Touch trackpad are minor updates, this year's 13-inch Retina Apple MacBook Pro is still an easy top pick for high-end ultraportables.... Read the full review on pcmag.com
Essentially, an Apple ID is your passport to the Apple ecosystem of hardware and software. It is an important information that smooths the interaction between you, your hardware, software and services that Apple provides.
The iMessages are related to your Apple ID. Therefore, any device that is setup to use your Apple ID is capable of receiving your iMessages. If you have an iPhone, the phone number is also associated with your Apple ID and all devices using the Apple ID can receive iMessages sent to your phone number.