Most PCs come with this port, traditionally used for connecting printers, and sometimes hobbyist kits like our Simple Parallel port LED Board. Some newer ones have however done away with this with the rise of the USB printers.
It uses (as the name implies) a Parallel Data Stream.

Parallel Port Pins:

Pin No Name Direction In/out Register Hardware Inverted
1 nStrobe In/Out Control Yes
2 Data 0 Out Data
3 Data 1 Out Data
4 Data 2 Out Data
5 Data 3 Out Data
6 Data 4 Out Data
7 Data 5 Out Data
8 Data 6 Out Data
9 Data 7 Out Data
10 nAck In Status
11 Busy In Status Yes
12 Paper-Out / Paper-End In Status
13 Select In Status
14 nAuto-Linefeed In/Out Control Yes
15 nError / nFault In Status
16 nInitialize In/Out Control
17 nSelect-Printer / nSelect-In In/Out Control Yes
18 - 25 Ground Gnd


In the table - when hardware inverted is yes, it means that when you write a 1 to this pin, it will output a 0, and vice versa.

You can learn to connect things to this, and program this port at our Simple Parallel port LED Board article.

Jan Axelson's Parallel Port FAQ is somewhat helpful if you are having trouble with this.
Interfacing to The IBM-PC Parallel Printer Port A nice resource with a few more projects to try.