I am new to PICs and would appreciate some help.  My question concerns the
use of resistors.  What is a "pull-up" resistor for, why and when do you
need it?  I see them used sometimes and not other times.  Can you connect an
I/O pin directly to 5 volts and read a logic high or is a resistor of some
kind needed. Can you connect an I/O pin directly to another I/O pin or to a
pin from other TTL chips, transistor etc.?  Is there some rule of thumb that
I don't know about?  I know my ignorance is bare naked here, but bear with me.

Thanks in advance.