For most people who don't ride or aren't familiar with horses and their tack, the main difference between the two disciplines is the saddle.
Being a Brit, I learnt to ride English style, on an English saddle, but I have always been interested in the Western style of riding which is now gaining in popularity in this country.
A back cinch is sometimes used to balance the saddle when roping cattle, or for showing, but is not necessary for general pleasure or trail riding. It ishould never be fastened tightly, its purpose only being to prevent the saddle tipping forward.
Wheras with an English saddle, a light saddle pad or numnah may be used mainly to protect the saddle from dirt and stains, a thick saddle pad is always used with a Western saddle to protect the horse, since the nice mock sheepskin lining of the western saddle is purely cosmetic and there is no actual padding to protect the horse. These pads can be as plain or colorful as the rider wishes and also come in a variety of shapes and types.
A synthetic is especially useful for endurance riding, because of the lighter weight and their ability to shrug off mud and water. I use a synthetic Western saddle myself and just love it.
Next time I'll touch on some of the actual differences between Western and English riding.