Serial protocol

UART can be used to connect 2 nodes using 2 wires - control and data. The control like start and stop transmission between the nodes. 

I2C is from Philips. It is a master slave set up on a bus. The node got control of the bus becomes the master. It sent out the address of the slave it wants to communicate with, the node (read or write) and follow by data transmitting in the wire. The address are configured in each device connoted to the bus either built in or via jumpers etc  

SPI came from Motorola. It is also a master slave set up but the master is usually a more intelligent controller and slaves are less intelligent peripheral devices. Master select the space via a CS (chip select) wire that connect to a specific slave. Therefore the master controller may have more than 1 CS wire to multiple slave. Master is also responsible to generate the clock signal to support the transmission (sampling the wire for data). There is also a daisy chain confirmation possible to allow the master to communicate to multiple slaves via one CS wire but the implementation is complex, data need to pass through all slave sequentially during transmission and not all device support this configuration. There are 2 data wires. One to transmit era from master to slave and one for the reverse.