hi what is differnce between serial and software serial functions?
One uses hardware interrupts and timers. The other doesn't.
if i want to communicate with bluetooth module whic i must use?serial or software serial? also communicate with pc?
HardwareSerial is connected to pins 0 and 1 (Uno, Mega and Mini) and is used for communication with the PC and uploading programs.
If you are using an Uno or Mini with only 1 HardwareSerial it is usual to use SoftwareSerial to create another serial port for communication with the Bluetooth module.
You will get much better advice if you give full information - such as what Arduino you are using and what Bluetooth module you are using.
...R Serial Input Basics
Hardware serial uses a hardware uart (with its own timer / baudrate generator). Software serial is a software implementation and more limited.
For a simple module like the HC-05 you can use software serial and keep the hardware serial for uploading / debugging. Note that it's not advisable / useful to use software serial on pins 0 and 1.