1. High Level Input Voltage
this is the minimum voltage level required at an input for a logic 1.... if the input voltage falls below this point, it will not be considered a logic 1.
2. Low Level Input Voltage
this is the maximum voltage level required at an input for a logic 0.... if the input voltage rises above this point, it will not be considered a logic 0.
4. High Level Output Voltage TTL Loads
this is the minimum voltage level for a logic 1, at the output of the gate under load.
6. Low Level Output Voltage TTL Loads
this is the minimum voltage level for a logic 0, at the output of the gate under load.