OK the cheapest consumer zoom lenses have variable zoom lengths and non-internal focusing. They basically consist of 2 concentric tubes. This means that when you zoom, the lens either lengthens or shortens. And when you focus, the front element will rotate.
The disadvantages are that
1. With a rotating front element, use of polarizing lenses will be affected, because the polarizer needs to be at a particular orientation to work best. If changing the focus point causes it to rotate, this can be a big headache. Internal focusing takes care of this problem.
2. With a lens that changes length, every time you zoom, air will be sucked into or pushed out of the lens. This will be accompanied by dust, of course, which means that over the long term, dust will invariably accumulate inside these types of lenses. A fixed length zoom (such as Canon's 70-200 f2.8L) remains the same length no matter the zoom length, so will not have that problem.
Most if not all fixed-length zooms will be internally focusing, but the opposite may not be true - you may have variable-length but internal focusing lenses.