David Bell • Link
Perhaps the most obvious distinction between the 17th Century guitar and those of modern times is that it was a five-string instrument, similarly tuned to today but missing the bottom E.
Often, they were more properly a five-course guitar, with ten strings in pairs and the pairs variously tuned in unison and octave, in much the same way as a modern 12-string instrument.
At this time, guitar music was written in tablature, or as chords indicated by letters and diagrams, not so different in essence to today. Strumming and plucking were both used by players of even the highest skill.
An interesting site is http://www.classicalguitarmidi.com/history/ which also has pictures of two 17th Century guitars.