Like was previously said, Sinking inputs are most common. They tend to be easily visualized. You put 24vdc on one side of switch, close the switch and it sends 24vdc to an input. Voltage High= Input ON
A sourcing input is kind of opposite. You put DC COM on one side of a switch, close the switch and the Input is pull down to DC COMM. Voltage LOW =Input ON.
Many PLC inputs can be configured for Sinking or Sourcing. The trick is that the inputs will have an extra terminal that determines if it is sinking or sourcing. For instance, on micrologix 1400, you will have a COM0, COM1, COM2 and a COM3.
COM0 is used for inputs 0-3
COM1 is used for inputs 4-7
COM2 is used for inputs 8-11
COM3 is used for inputs 12-19
This allows you to use some as Sinking and some as Sourcing.
For example- If I have a mix of NPN and PNP sensors, I will need to use different sets of inputs. If sensor 1 is a NPN proxy switch (ON or OFF), I would connect 24vdc to COM0 and then connect the sensor to Input 0. This would be a sourcing input.
If sensor number 2 is PNP, I can not put it on the same bank of inputs as sensor 1. I must move it to a new bank of inputs. So I can now hook up DC COM to COM1 and connect the PNP sensor to input 4. This is a sinking input.
To say it differently, a sinking input will provide a path to ground for a sensor whose output is High when ON. Current will flow from the sensor, to you sinking input and out through the DC Com that was connected to the COM of that input.
A sourcing input will provide a path from the 24vdc on the COM terminal of the input. When the input is grounded, current will flow out of the input to ground provided by the sensor.
In both cases, the actual sensing of the Input being ON or OFF is simply seeing if a current is flowing though the input terminal. If it is a Sourcing input, the current is actually leaving the plc to the sensor. If the input is a Sinking input, the current is flowing from the sensor to the plc.
All of the above are for discrete inputs. Either ON or OFF. Analog is a little different deal.