The drive of a tool (e.g. nut-driver) is usually a hollow square (solid hexagon for screwdriver bits).
To operate this tool, you need a tool with a suitable output (a solid square for a nut-driver; a hollow hexagon for a screwdriver bit).

In the case of a screwdriver or spanner, the drive is your hand, i.e. the handle where you hold the tool.

The output of a tool is, for example, the (solid) square on a reversible ratchet, on a torque spanner or on an impact wrench.
For e.g. screwdrivers, we understand the output to be the tip of the screwdriver profile. 
In the case of a spanner or nut-driver, the output is the part that is placed on the nut or screw

The input and output must fit together to prevent damage to the tool or screw. You can find an overview of the different screw profiles here

Please also note that a non-insulated nut-driver (e.g. 900-17 and a reversible ratchet with protective insulation (e.g. 916KV) have compatible input and output. However, they must never be used in this combination on live parts; the reverse case (insulated nut-driver and non-insulated reversible ratchet) is also not permitted.

Also note: please do not combine a nut-driver for manual operation with an impact wrench