The hive design is very simple. Many people have used drawers, crates, and even old cylindrical containers. Almost anything can be converted into a top bar hive as long as it can be weather proofed and a good entrance can be provided.
Instead of regular frames a TBH uses 'top bars'. Basically, these are just pieces of wood about 1 3/8" wide. The bees build their comb directly from these. No sheets of foundation wax are used though some may use small strips of foundation wax as guides for the bees. It is wise for all of the hives in a bee yard to be matched in size so the top bars are interchangeable. However, TBHs do not have to match the exact dimensions of a Langstroth hive.
Types of TBHs
The two main designs are Kenyan and Tanzanian. The Kenyan style hive has sloped sides. This is to take advantage of the bees building hexagonal shaped cells. The sloped sides force the bees to slope their honeycomb as well. This makes the comb lighter over it's length so it won't break off as easily and supposedly helps keep the bees from attaching their comb to the sides. One of the disadvantages of the Kenyan style is that you cannot put regular frames into it. This would keep someone from transferring a traditional nuc directly to the hive and from sharing frames of brood or honey from a traditional hive.
The Tanzanian TBH has straight (vertical sides). The advantages of the Tanzanian are that you can build them to the dimensions of a Langstroth hive thus allowing them to take frames from your existing hives. You can also transfer nucs. The disadvantages are more comb per top bar possibly encouraging breakage and the possibility of more comb attachment. There is some disagreement if there is more comb attachment or not using the Tanzanian design. For my own hives I have decided to go with the Tanzanian design so I can transfer to and from medium-sized supers.
Ease of Use
Unlike traditional hives there is no heavy lifting involved as there are no honey supers. Also, no bees are exposed except where a section of comb is removed since the top bars completely touch across the top, unlike frames in Langstroth hives. Many hobby beekeepers have switched to TBHs for this reason alone.
Bees will naturally build a smaller cell size than they are forced to by the usual foundation wax that is used in commercial beekeeping. This gives many health advantages to the bees, especially against varroa and tracheal mites. I'll touch on this in more detail in later articles.
For more information on TBHs and other beekeeping ideas check out Michael Bush's website.