No, UART and RS-232 are not the same.

UART is responsible for sending and receiving a sequence of bits. At the output of a UART these bits are usually represented by logic level voltages. These bits can become RS-232, RS-422, RS-485, or perhaps some proprietary spec.

RS-232 specifies voltage levels. Notice that some of these voltage levels are negative, and they can also reach ±15V. Larger voltage swing makes RS-232 more resistant to interference (albeit only to some extent).

A microcontroller UART can not generate such voltages levels by itself. This is done with help of an additional component: RS-232 line driver. A classic example of an RS-232 line driver is MAX232. If you go through the datasheet, you'll notice that this IC has a charge pump, which generates ±10V from +5V.

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Bibliography:

• RS-232 vs. TTL Serial Communication - SparkFun Electronics
• communication - USART, UART, RS232, USB, SPI, I2C, TTL, etc. what are all of these and how do they relate to each other? - Electrical Engineering Stack Exchange
• Serial Programming/Typical RS232 Hardware Configuration - Wikibooks, open books for an open world

Serial is an umbrella word for all that is "Time Division Multiplexed", to use an expensive term. It means that the data is sent spread over time, most often one single bit after another. All the protocols you're naming are serial protocols.

UART, for Universal Asynchronous Receiver Transmitter, is one of the most used serial protocols. It's almost as old as I am, and very simple. Most controllers have a hardware UART on board. It uses a single data line for transmitting and one for receiving data. Most often 8-bit data is transferred, as follows: 1 start bit, low level, 8 data bits, 1 stop bit, high level. The low level start bit and high level stop bit mean that there's always a high to low transition to start the communication. That's what describes UART. No voltage level, so you can have it at 3.3 V or 5 V, whichever your microcontroller uses. Note that the microcontrollers which want to communicate via UART have to agree on the transmission speed, the bit-rate, as they only have the start bit's falling edge to synchronize. That's called asynchronous communication.

For long distance communication (a that doesn't have to be hundreds of meters) the 5 V UART is not very reliable, that's why it's converted to a higher voltage, typically +12 V for a "0" and -12 V for a "1". The data format remains the same. Then you have RS-232 (which you actually should call EIA-232, but nobody does.)

Important: RS-232 vs. TTL Serial Communication - SparkFun Electronics

The timing dependency is one of the big drawbacks of UART, and the solution is USART, for Universal Synchronous/Asynchronous Receiver Transmitter. This can do UART, but also a synchronous protocol. In synchronous there's not only data, but also a clock transmitted. With each bit a clock pulse tells the receiver it should latch that bit. Synchronous protocols either need a higher bandwidth, like in the case of Manchester encoding, or an extra wire for the clock, like SPI and I2C.

Serial Programming/Typical RS232 Hardware Configuration - Wikibooks, open books for an open world

RS232      +-----------+   +-----------+   +-----------+   +-----------+
Interface  | Line      |   |           |   | Interface |   |           |
-----------+ Driver /  +---+   UART    +---+ Logic     +---+    CPU    |
           | Receiver  |   |           |   |           |   |           |
           +-----------+   +-----+-----+   +-----+-----+   +-----------+
                                 |               |
                                 |               |
                           +-----+-----+         |
                           | Baud Rate |         |
                           | Generator +---------+
                           |           |
                           +-----------+

The UART (universal asynchronous receiver transmitter) is the heart of the serial hardware. It is a chip or part of a chip with the purpose to convert between parallel data and serial data. RS-232 UARTs also typically add the necessary start/stop and parity bits when transmitting, and decode this information when receiving.

A UART typically operates entirely on computer logic voltage. Its serial data input/output voltage is the computer logic voltage, not the serial line voltage. They leave the actual line interface to a particular line driver / receiver. This line driver / receiver does not necessarily need to be an RS-232 line driver / receiver, but could e.g. also be an RS-422 differential driver / receiver. This, and the fact that baud rate, parity, number of stop bits, number of data bits are programmable is the reason why UARTs are called universal. The distinction between UART and line driver / receiver blurs if they are both placed in the same chip. Such chips are typically also sold under the label 'UART'.

UARTs are called asynchronous, because they don't use a special clock signal to synchronize with the the remote side. Instead, they use the start/stop bits to identify the data bits in the serial stream.

Thanks to the UART the rest of the hardware, as well as the software application can deal with normal bytes to hold the communication data. It is the job of the UART to chop a byte into a series of serial bits when sending, and to assemble series of bits into a byte when receiving. UARTs typically contain eight bit wide receiver and transmission buffers. Of which not all bits might be used if e.g. a 7 bit transmission is used. Received serial data is provided in parallel in the receiver buffer, to-be-send data is written in parallel to the transmission buffer. Depending on the UART the buffers might just have a depth of one byte, or a few bytes (in the range of 15 or 16 bytes). The less deep the buffers are, the more precise the communication with the CPU needs to be. E.g. if the receiver buffer just has a depth of one byte, and the data is not fetched fast enough, the next received data can overwrite the previously received data in the buffer, and the previously received data is lost.

Because of the fact that the timing on the serial interface is important, UARTs are typically connected to a baud rate generator, either an internal one in the UART chip, or an external one.

SPI (Serial Peripheral Interface) is another very simple serial protocol. A master sends a clock signal, and upon each clock pulse it shifts one bit out to the slave, and one bit in, coming from the slave. Signal names are therefore SCK for clock, MOSI for Master Out Slave In, and MISO for Master In Slave Out. By using SS (Slave Select) signals the master can control more than 1 slave on the bus. There are two ways to connect multiple slave devices to one master, one is mentioned above i.e. using slave select, and other is daisy chaining, it uses less hardware pins(select lines), but software gets complicated.

I2C (Inter-Integrated Circuit, pronounced "I squared C") is also a synchronous protocol, and it's the first we see which has some "intelligence" in it; the other ones dumbly shifted bits in and out, and that was that. I2C uses only 2 wires, one for the clock (SCL) and one for the data (SDA). That means that master and slave send data over the same wire, again controlled by the master who creates the clock signal. I2C doesn't use separate Slave Selects to select a particular device, but has addressing. The first byte sent by the master holds a 7 bit address (so that you can use 127 devices on the bus) and a read/write bit, indicating whether the next byte(s) will also come from the master of should come from the slave. After each byte receiver must send a "0" to acknowledge the reception of the byte, which the master latches with a 9th clock pulse. If the master wants to write a byte the same process repeats: the master puts bit after bit on the bus and each time gives a clock pulse to signal that the data is ready to be read. If the master wants to receive data it only generates the clock pulses. The slave has to take care that the next bit is ready when the clock pulse is given. This protocol is patented by NXP(formerly Phillips), to save licensing cost, Atmel using the word TWI(2-wire interface) which exactly same as I2C, so any AVR device will not have I2C but it will have TWI.

Two or more signals on the same wire may cause conflicts, and you would have a problem if one device sends a "1" while the other sends a "0". Therefore the bus is wired-OR'd: two resistors pull the bus to a high level, and the devices only send low levels. If they want to send a high level they simply release the bus.

TTL (Transistor Transistor Logic) is not a protocol. It's an older technology for digital logic, but the name is often used to refer to the 5 V supply voltage, often incorrectly referring to what should be called UART.

About each of these you can write a book, and it looks I'm well on my way. This is just a very brief overview, let us know if some things need clarification.