Some devices do not have any actual output VOLTAGE, such as the LM339 comparator IC. It "looks" like an op-amp, but it is a comparator. The output of any of the 4 comparators is always either grounded or open (high resistance from ground). If no PULL-UP resistor is present, then when the device is "NOT grounded" its output pin will not rise to VCC. A high value resistor is connected from +vcc to the output of the comparator. When the output is UNGROUNDED, you get a +vcc reading. OP-AMPS such as the 741 or Lm324 have actual output voltages and therefore do not need the "pull-up" resistor on the output pin.
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Pull up generally means connecting the PIC input to Vcc (often 5volts)Pull down generally means connecting the input to ground (often 0 volts or the negative line)Typical values may be around 10K ohms.When the input is taken high or low by the signal the resistor prevents too much current passing into the signal line BUT keeps the input at a predetermined state- ie. high or Low.
Logic gates need to have definite states even if unconnected. Floating states can cause errors. Pull up resistors cause these gates to have the correct state.Click this for a great explanation.