Probe
Probes are spacecraft built around probe cores (unmanned command modules). They can be used to control rockets and other systems, 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 still usually much lower than that of a manned vessel with comparable abilities. 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.
