I thought it might be helpful to explain the difference between the lute, the theorbo and the archlute, all mainstays of baroque and Renaissance music.
The lute is the "base instrument" for the other two. Wikipedia defines the lute as "any plucked string instrument with a neck and a deep round back enclosing a hollow cavity, usually with a sound hole or opening in the body. It may be either fretted or unfretted ... The European lute and the modern Near-Eastern oud descend from a common ancestor via diverging evolutionary paths."
The theorbo was developed during the Baroque era to provide a stronger bass voice to the instrument, particularly for use in early opera and other genres. Wikipedia notes that "A theorbo differs from a regular lute in that the theorbo has a much longer neck which extends beyond the regular fingerboard/neck and a second pegbox at the end of the extended neck. Low-register bass strings are added on the extended neck. This gives a theorbo a much wider range of pitches (notes) than a regular lute."
The archlute is a sort of mid-size compromise between the smaller lute and the larger theorbo. Wikipedia calls it "Essentially a tenor lute with the theorbo's neck-extension".
To illustrate the differences in size, I found this image in several locations on the Internet. I've no idea who owns the copyright for it, and I'll gladly provide that attribution if the owner cares to get in touch.
Of course, the instruments sound very similar, with stronger bass notes from the larger archlute and theorbo. Here's a selection of videos of both being played. The first two provide introductions to the instruments, after which there are several short pieces. Enjoy!
Here's an interesting combination: an archlute and a salterio, a zither-type instrument.
Now for some theorbo pieces.
And here's one played by both archlute and theorbo, to highlight the aural and visual differences between the instruments.