Hi Leo,

8051 is bit/byte processor the 8051 ports can be accessed as individual pins(bits) or as a whole port(byte) you can use the said configuration. There exists instruction to manipulate ports in bit as well as byte format.
Exaple:
Reading:
Bit -> mov c,p1.0
byte-> mov a,p1
Writing:
Bit -> mov p1.0,c
byte-> mov p1,a

You need to read the "bible" links given by erik.