This is the original parallel port, it's been used on all PC computers since the very beginning and has mostly remained unchanged over the years. When IBM came up with the first PC, they opted to go along the lines of the the most prominent printer manufacturer at the time, Centronics. This printer manufacturer had developed a set of control signals that was up to the task of controlling computer printers and since many printer manufacturers had been adopting this now standard design, it was the obvious choice to make.
But when IBM constructed it's PC they opted to not use the true Centronics connector which was a 36 conductor Amphenol connector (also known as the Centronics connector). IBM opted for a 25 pin D shell connector also called a DB-25 connector. Since then, printer manufacturers have always used Centronics connectors and PC manufacturers have been using DB-25 connectors. This is the reason why you need this special adapter cable that is known as a printer cable and is now a standard accessory.
Following are pictures of the connectors pin view and it's signal assignments, also are a description of each signal.

Each signal is identified by its pin number on a DB-25 and Centronics 36 pin connector and its signal name.

Strobe

The strobe line is the heart of the parallel port, it tells the printer when to sample the information of the data lines, it is usually high and goes low when a byte of data is transmitted. The timing is critical for the data to be read correctly, all bits on the data lines must be present before the strobe line goes low, to insure data integrity when the printer samples the data lines. The time needed for each byte is about half a microsecond then the the strobe line goes low for about one microsecond and then the data is usually still present for another half microsecond after the strobe goes high. So the total time needed to transmit a full byte is around two microseconds.

Data

These 8 lines carry the information to be printed and also special printer codes to set the printer in different modes like italics, each line carries a bit of information to be sent, the information here travels only from the computer to the printer or other parallel device. These lines function with standard TTL voltages, 5 volts for a logical 1 and 0 volts for a logical 0.

Acknowledge

This line is used for positive flow control, it lets the computer know that the character was successfully received and that it's been dealt with. It's normally high and goes low when it has received the character and is ready for the next one, this signal stays low for about 8 microseconds.

Busy

As seen above (strobe line), each byte takes about 2 microseconds to be sent to the printer, this means the printer is receiving about 500,000 bytes per second (1 sec divided by 2 microseconds), no printer can print this fast, so they came up with a busy line. Each time the printer receives a byte this line will send this line high to tell the computer to stop sending, when the printer is done manipulating the byte (printing, putting it in the buffer or setting it's internal functions) it then goes back low, to let the computer know that it can send the next byte.

Paper End

Also refered to as Paper Empty, this line will go high when you run out of paper, just like the paper out light on your printer, this way the computer will know and can tell you of the problem. When this happens the busy line will also go high so the computer stops sending data. Without this line when you would run out of paper the busy line would go high and the computer would seem to be hanged.

Select

This line tells the computer when it is selected (or online), just like the light on your printer. When the select line is high the printer is online and is ready to receive data, when it's low the computer will not send data.

Auto Feed

Not all printers treat the cariage return the same way, some will just bring the print head to the beginning of the the line beeing printed and some will also advance the paper one line down (or roll the paper one line up). Most printers have a DIP switch or some other way to tell your preference of how to interpret the cariage return. The auto feed signal lets your computer do the job for you, when it put's this signal low, the printer will feed one line when it gets a cariage return, by holding the signal high the software must send a line feed along with the cariage return to obtain the same effect.

Error

This is a general error line, there is no way of knowing the exact error from this line. When no errors are detected, this line is high, when an error is detected it goes low. Some of the errors that can arrise through this line are: cover open, print head jammed, a broken bealt by detecting that the head does not come back to it's home position or any other error that your printer can detect.

Initialize Printer

This line is used to reinitialize the printer, the computer will accomplish this by putting the line, wich is normally high, to it's low state. This is very useful when starting a print job, since special formating codes might have been sent to the printer on the last job, by reinitializing the printer you are sure of not messing up the whole thing, like printing the whole document in italics or something.

Select Input

Many computers give the option of letting the computer the option of putting the printer online or not, by putting this signal high the printer is kept in it's offline state and putting it low the printer is online and will accept data from the computer. Many printers have a DIP switch to let decide if the computer can control the online state, when the switch is active it will keep this line always low, thus keeping the computer from putting the printer offline.

Ground

This is a regular signal ground and is used as a reference for the low signal or logical 0.