new in ABAP

Hello Everybody

what is the difference between pack and floating point datatypes, i ABAP Objects book, there is mentioned that if you are concious about accuracy then use pack datatype, otherwise use floating point. i write a program

data p1 type p.

p = 3 / 4.

write p. the result of this statement is 0,75

data f1 type f.

f1 = 3 / 4.

write f1 and the result of this is statement is 7,5000000E

so when we use pack number and what floating point give me example in detail and clear

Thanks

Rai