Are we all thoroughly confused yet?!
If not, let me help:

The internal UART is a Parallel in, serial out device when used for output, when used for input it is a serial in, parallel out device.

By CMOS verbage the internal UART is a serial port, by PC verbage it is not.

External UARTs (rarely used in '51 projects) are usually parallel interface to the processor, some may use serial processor interface such as IIC or SPI.

Erik

another note:
using AT89c51, i dont think that there is any serial port in it.
Farhan, I hate to do this, but in this respect I need to ask you to go through the extreme drudgery of having a look at the datasheet.