"Thiele" refers to A. Neville Thiele, who, along with another researcher named Small, came up with the Thiele/Small (or T/S) parameters used for measuring loudspeaker performance.
Thiele also created a variety of speaker cabinet designs for audio usage, including the TL806 and TL606. The TL806 is a 12" ported speaker cab, which is what we're referring to here. Mesa calls it the "Thiele" cab and the name stuck. The TL606 is a 15" version.
You can build Thiele-style cabs in 1x12, 2x12, and 4x12 (and up) configurations. I like the 1x12 because it's loud, portable, and sounds very big for its size due to the porting, internal damping, and design. However, the sound and response (especially in the low end) is going to be different from your standard sealed cab, which means that some will not like it.