Raspberry Pi is the credit-card sized embedded computer system that is useful mostly for developers and engineers where they can program stuffs and run on Raspberry Pi devices but there are other purposes if you’re not a developer like turning one device into a media center, server and other stuff you want the Raspberry Pi to act as. The CPU and GPU are obviously different but useful for embedded devices and the OS is whatever Linux based distribution you can run on. There’s also the default OS along with default packages for Raspberry Pi devices as well as the hardware addons like the touch screen monitor, casing for the device, breadboard for whatever engineering stuff you want, etc.
Raspberry Pi 2 has support for Sailfish OS, Snappy Ubuntu Core, Windows 10 IOT and the list goes on. The Raspberry Pi devices may be the definition of IOT but speaking of IOT, there may be chances that they may be more vulnerable to cyber-penetration that you may need to find some services or apps to secure the devices.
Unfortunately for Raspberry Pi Zero, although it may be faster than the original Raspberry Pi, as the title suggests, it has some of the features missing from both the original Raspberry Pi and Raspberry Pi 2 such as the lack of Ethernet port and standard USB ports, and some parts of the system specs are inferior if you compare Raspberry Pi Zero to Raspberry Pi 2.