pin "high" = voltage close to i/o supply ("vdd" or "vcc")
pin "low" = voltage close to ground ("vss" or "gnd")
(The exact meaning of "close to" will vary.)


driven high (output enabled, output high, internal pull-up/down irrelevant):
- tries very hard to make voltage on pin high
- current flows to the pin (from vdd) as necessary until the pin is high (or something breaks)
- for BBB:  voltage will be at least 2.85V on condition that current does not exceed 4 mA

driven low (output enabled, output low, internal pull-up/down irrelevant):
- tries very hard to make voltage on pin low
- current flows from the pin (to vss) as necessary until the pin is low (or something breaks)
- for BBB:  voltage will be at most 0.45V on condition that current does not exceed 4 mA

pulled up (output disabled, internal pull-up enabled):
- weakly tries to make voltage on pin high
- a small amount of current flows to the pin (from vdd) until the pin is high
- for BBB:  max current is 0.243 mA

pulled down (output disabled, internal pull-down enabled):
- weakly tries to make voltage on pin low
- a small amount current flows to the pin (to vss) until the pin is low
- for BBB:  max current is 0.11 mA

floating (output disabled, internal pull-up/down disabled):
- also known as "high-impedance" or "high-Z"
- tries to avoid having any influence on the voltage on the pin
- very little current flows to/from the pin
- for BBB:  max current is 0.018 mA


input characteristics for BBB:
- pin will be regarded as low when input is disabled or pin voltage is at most 0.8V
- pin will be regarded as high when input is enabled and pin voltage is at least 2V