Click Here for pictures. Serial connectors are small rectangular connectors on the rear of the case with 9 pins. They consist of 2 rows of pins, one with 4 pins, the other with 5 pins. In a serial connector on the back of the case, you see the actual pins. Click Here for pictures.
All computers come with at least one serial port.
Parallel connectors are a little larger rectangular connectors with 25 pins, one row with 12, one with 13. On a parallel connector on the case, you see the holes that the pins plug into. Click Here for pictures. All computers come with at least one Parallel Port. USB connectors are supplied on all computers sold in the last few years.