19 - Coax and CAT5
Why coax and why CAT5? Is one better than the other?
Coax is shorthand for Co-axial. In other words a small wire in the center, surrounded by a insulator, which is surrounded by a shield, and more insulation. Coax is described as single ended transmission line, or unbalanced. The shield is typically tied to system ground, and the signal is carried as a voltage on the center wire. In the video world, Coax is usually 75 ohms impedance.
CAT5 is shorthand for Category 5. Other common cables are CAT3, CAT5e, and CAT6. CAT5 became super popular because it is the minimum requirement for 100baseT Ethernet networking. Internally it is four twisted pairs of small gauge wire, wrapped in a PVC jacket. 
The jacket is not shielded, so it is also call Unshielded Twisted Pair, or UTP. CAT5 pairs are 110 ohms impedance. Twisted pairs are described as differential transmission lines, or balanced. Typically a system would see the signal over a twisted pair as the difference between the pairs, rather than the absolute voltage of any one of the wires.
To say one is better than the other would be misinformed. Each has advantages and disadvantages depending on the job at hand.
Coax has superior control over keeping a consistent impedance, and can reach ultra high bandwidths. However noise rejection is dependant on shielding the center wire from outside signals.
CAT5 can have multiple pairs in a small cable, and can have superior noise rejection. However CAT5 impedance can vary quite a bit from ideal, and the four pairs can vary in electrical length, all of which result in a limitation in the practical bandwidth of CAT5.
Why each type of cable is used today for various things like voice, video and data has a lot more to do with decades of legacy rather than engineered solutions.
Some interesting things are happening with CAT5 in the last couple years. Since it is a cost effective cable, and it is easy to install and field terminate, there have been many accessory products offered to convert just abut anything to CAT5. There are both active circuits and passive baluns to convert a single ended audio and video signals into differential pairs. These boxes typically have either an RJ45 or a punch down block, and can make routing audio and video through the home cheap and easy. The only problem with this so far is there is no standard for doing this, so brand A’s CAT5 solution will probably not be compatible with brand B’s solution.