A baseband signal has significant spectral content near zero frequency. A digital pulse sequence or an audio waveform before frequency translation can be treated as baseband.

A passband signal occupies a frequency band centered around a nonzero carrier frequency $f_c$.

Baseband and passband describe where a signal's spectrum lies, not whether the information is analog or digital. Digital symbols can be transmitted directly as baseband pulses or represented by changes in a high-frequency carrier.

Moving information between baseband and passband is frequency translation. The message spectrum keeps its internal structure while being relocated to another frequency range.

Passband operation can match the useful frequency range of a physical channel or let several transmissions occupy different frequency bands.

The distinction is representational: a receiver commonly translates an arriving passband waveform back toward baseband before extracting the information it carries.