The virginal may be regarded as a harpsichord with only one string per pitch, as opposed to the harpsichord which has more than one string per pitch. The shape of virginals is also more like a rectangular box, while the harpsichord has a shape more similar to the modern concert piano.
The virginal has a large number of strings tuned at fixed pitches, with a keyboard used to trigger the strings.
The virginal came in use in the 1400s, but its origin is unclear. The later spinet (of the 1600s) may be regarded as a virginal with more than 1 string per pitch, and a more interestingly shaped soundbox.
Source: http://www.miayf.org/keyboard/virginal/