Probes are craft whose command modules have zero crew capacity, modules generally known as “probe cores”. Craft controlled by probe cores that include crewed modules lacking command capability (the HSC or the Mobile Lab) are automatically typed by KSP as probes, although players may consider them some other type of craft. Likewise, players may consider particularly large probes or probes resembling conventional planes, rockets, or rovers as belonging to other types or categories.
They can be used to perform experiments, and to provide other functionality, all controlled remotely, where Kerbanauts could not or should not go.
Even though probe cores constantly consume electricity and thus usually need more batteries and energy-sources than manned command pods, the mass of a probe is much lower than that of a manned vessel with comparable abilities in most cases. A lower payload mass means that less fuel is required to bring it to its destination. Also, some players have ethical issues with abandoning Kerbals while they do not worry about abandoning an unmanned probe. Not requiring the fuel for the return trip further increases the range of the vessel.