The " SoftwareSerial " library has been developed to allow serial communication on other digital pins of the Arduino, using software to replicate the functionality of  built-in chip called UART

Serial Peripheral Interface (SPI) is a synchronous serial data protocol used by microcontrollers for communicating with one or more peripheral devices quickly over short distances. 

I2C combines the best features of SPI and UARTs. With I2C, we can connect multiple slaves to a single master (like SPI) and we can have multiple masters controlling single, or multiple slaves.

An interrupt is a signal that tells the processor to immediately stop what it is doing and handle some high priority processing.  That high priority processing is called an Interrupt Handler.