Hi Bikash
When you transmit data in binary format, at the receiving end you receive it in a buffer, to display it you need to convert it to ascii otherwise it would be displayed garbage if displayed at all.
example:
if u have 8 bit adc reading a voltage you transmit the measured value in binary from 0 - FF, when the serial port receives it you cant display it as is, you need to convert it into ASCII. Take the upper nibble first convert it to ascii value display it, take the lower nibble, convert it to ascii and display it again.
text is always ascii while real values are always binary(hex, oct, dec etc... same thing but hex is easiest to work with coz u can convert upper nibble and lower nibble seperately and u display the correct answer while if u need to work with decimal u need 3 digits to display the binary byte then you need to do BCD - binary coded decimal-)

Regards
mahmood