What are I2C, SPI, and UART, and which one should I use?

These are three common ways a microcontroller talks to other chips, like sensors, displays, and modules. They are not "better" or "worse" overall, they are different tools.

I2C (two wires, many devices)

Wires: SDA + SCL (plus power and ground)
Best for: sensors and small peripherals
Pros: multiple devices can share the same two signal wires
Cons: slower than SPI, and wiring/noise can matter on longer runs

SPI (fast, more wires)

Wires: SCK, MOSI, MISO, plus a separate CS pin per device
Best for: fast displays, SD cards, high speed sensors
Pros: fast and reliable
Cons: more wires, and each device needs a chip select pin

UART (classic serial)

Wires: TX and RX (plus power and ground)
Best for: GPS modules, Bluetooth modules, debugging, talking to another board
Pros: simple, widely supported
Cons: usually point-to-point (not many devices on the same two wires)

Which one should you choose?

Choose I2C for most sensors and small modules. It is the easiest wiring.
Choose SPI when you need speed (displays, SD cards).
Choose UART when a module specifically says "TX/RX" or "Serial", or when connecting two devices directly.

Bottom line

I2C is simplest for multiple sensors, SPI is fastest, and UART is the classic two-device serial link. Pick the protocol your module supports, then choose based on wiring and speed needs.