What is RS232? RS432? RS485? MIDI? What do these have in common?

(Note: "RS432" in the question is almost certainly a typo for RS-422, the real EIA/TIA standard; it is answered as RS-422 below.)

RS-232

• A single-ended (unbalanced) serial standard where each signal is referenced to a common ground. Logic levels are bipolar voltages: a mark/idle is roughly −3 to −15 V and a space is +3 to +15 V (inverted relative to TTL). A transceiver such as the MAX232 converts between UART TTL levels and RS-232 voltages.
• Point-to-point, short distances (~15 m at low rates), one driver and one receiver. Classic 9-pin (DB-9) PC serial port; signals include TX, RX, RTS, CTS, DTR, DSR, etc.

RS-422

• A differential (balanced) standard: each signal is carried on a twisted pair, giving strong common-mode noise rejection, long cable runs (~1200 m), and high data rates.
• Topology is point-to-multipoint on the receive side: one driver can fan out to multiple (up to ~10) receivers, but only a single driver per line. Often used for long-haul or noisy industrial links.

RS-485

• Also differential, but designed for multi-drop / multi-point networks: multiple drivers and receivers (typically up to 32 unit loads, more with modern transceivers) share the same bus. Supports half-duplex (two wires, drivers must take turns and tri-state when idle) or full-duplex (four wires).
• The physical layer behind many industrial/building protocols (Modbus RTU, Profibus, DMX512).

MIDI (Musical Instrument Digital Interface)

• A current-loop serial interface (~5 mA loop) running at a fixed 31,250 baud, opto-isolated at the receiver to break ground loops between instruments. Uses standard UART framing (8N1).
• Carries musical event messages (note on/off, velocity, control change, etc.) between synthesizers, controllers, and computers.

What they have in common All four are serial communication standards that move data one bit at a time. RS-232 and MIDI in particular are asynchronous, UART-framed serial links (start/data/stop bits, agreed bit rate) — they differ mainly in the electrical/physical layer (single-ended voltage vs. differential pair vs. current loop) and in topology (point-to-point vs. multi-drop). In other words, they are all variations on framing the same kind of asynchronous serial byte stream onto different wiring and signaling schemes.

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