UART is the protocol/peripheral layer, the asynchronous framing (start/data/parity/stop) at the MCU's logic voltage levels. RS-232 and RS-485 are physical-layer electrical standards that carry that framing over a cable. RS-232 is a point-to-point standard using bipolar voltages (nominally ±5 to ±15 V, with the polarity inverted relative to logic), good for short runs, and needs a level-shifting transceiver (like a MAX232) between the MCU's UART pins and the connector. RS-485 is a differential, multi-drop standard: it sends each bit as the voltage difference between two wires, which rejects common-mode noise and allows long cable runs (hundreds of meters) and many devices on one bus, but it's half-duplex on a two-wire bus, so you must control the transceiver's driver-enable to turn the line around between transmit and receive. So the same UART byte stream can ride over plain logic levels, RS-232, or RS-485 depending on the transceiver; the framing is identical, the electricals and topology differ. The practical danger is wiring an MCU's 3.3 V UART pins directly to an RS-232 port, the ±12 V will damage the MCU.
Communication Protocols · Interview question
What's the difference between UART, RS-232, and RS-485?
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From the lesson
UART
Two wires, no clock, and a pre-agreed baud: the UART frame (start/data/parity/stop), how the receiver recovers timing by oversampling, and an interrupt-driven RX ring buffer.