Almost everything is serial. RS232 is serial, USB is serial, UART is serial, I2S is serial, TWI is serial (different name for I2C), SPI is serial... ISP uses SPI to program the microcontroller so the programmer is serial (and that is the only way for ISP programming). The programmer will connect 2 devices using different serial modes to transfer the data. One device is the microcontroller that uses SPI and the other is the PC that can use the USB port, or the Serial port (RS232) or the paralel port. The programmer will use eighter a dedicated ICs, or tranzistors, or diodes or whatever, to convert the signal from one format to the other. So this is different of the regular serial communication with the PC that requires just the voltage level adjustment. For that, the easiest way to do it is using a MAX232 IC, but you can also use tranzistors. Does it make more sense now?