Input voltage range of opamps
In normal operation, V+ and V- are the same.  If they are different, the op-amp is being used as a comparator.
An important spec in choosing an op-amp is the "input common-mode voltage" range.  The voltages at the inputs must be in this range for proper operation.
Some op-amps are "RRIO" or rail-to-rail input/output.  As you might guess, this means the input common mode range goes from the Vee rail to the Vcc rail.
Some op-amps which include only the Vee rail in the common-mode range are referred to as "single-supply" op-amps.


Here is the spec in the LM324 datasheet: