YOu mean physically or purpose-wise?
COM ports come in two varieties- a DB-15 (I think it's 15) male (wide) and a DB-9 male. LPT ports are DB-25 female (Line PrinTer, I believe it stands for) are for, at least initially, Parallel communications in one direction with the printer. Later enhancements 9such as ECP and EPP) allow both bidirectional communication with devices as well as the exchange of state information between the PC and the Parallel device.
Parallel Ports and Serial Ports transfer data just as their names suggest. a Parallel port sends data 1 byte at a time, through 8 lanes, each representing a bit. a clock control signal determines the speed of this exchange, which is generally is quite low, compared to more modern interfaces.
Serial ports, while possessing many pins, only communicate a single bit at a time. a large number of the lines in a serial connection are unused, possibly integrated for the future expansion. the Serial transfer speed is generally quite low; usually even lower then Parallel ports, but have been bidirection since their inception. The main reason for such low transfer speeds lies in the nature of the serial ports controller chip- the UART chip, which comes in a few revisions. The limited nature of the early chips and the fact that the architecture must remain compatible means that in general the Serial port is one of the slowest external transfer interfaces that a PC has, and in the early days, the only one.
