An RS-232 interface has the following characteristics:
You probably already saw this connector on the back of your PC.
It has 9 pins, but the 3 important ones are:
Using just 3 wires, you can send and receive data.
Data is commonly sent by chunks of 8 bits (we call that a byte) and is "serialized": the LSB (data bit 0) is sent first, then bit 1, ... and the MSB (bit 7) last.
This interface uses an asynchronous protocol. That means that no clock signal is transmitted along the data. The receiver has to have a way to "time" itself to the incoming data bits.
In the case of RS-232, that's done this way:
Let's see how looks the byte 0x55 when transmitted:
Byte 0x55 is 01010101 in binary.
But since it is transmitted LSB (bit-0) first, the line toggles like that: 1-0-1-0-1-0-1-0.
Here's another example:
Here the data is 0xC4, can you see it?
The bits are harder to see. That illustrates how important it is for the receiver to know at which speed the data is sent.
The speed is specified in baud, i.e. how many bits-per-seconds can be sent. For example, 1000 bauds would mean 1000 bits-per-seconds, or that each bit lasts one millisecond.
Common implementations of the RS-232 interface (like the one used in PCs) don't allow just any speed to be used. If you want to use 123456 bauds, you're out of luck. You have to settle to some "standard" speed. Common values are:
At 115200 bauds, each bit lasts (1/115200) = 8.7µs. If you transmit 8-bits data, that lasts 8 x 8.7µs = 69µs. But each byte requires an extra start and stop bit, so you actually need 10 x 8.7µs = 87µs. That translates to a maximum speed of 11.5KBytes per second.
At 115200 bauds, some PCs with buggy chips require a "long" stop bit (1.5 or 2 bits long...) which make the maximum speed drop to around 10.5KBytes per second.
The signals on the wires use a positive/negative voltage scheme.
So an idle line carries something like -10V.