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Whitewater is formed in a rapid, when a river's gradient drops enough to form a bubbly, or aerated and unstable current. The term is also used loosely to refer to less-turbulent but still agitated flows.
Three factors separately or in combination can create rapids: gradient, constriction, and obstruction.
The gradient of a river is the rate at which it loses elevation along its course. This determines the river's slope, and thus (to a large extent) its rate of flow.
Constrictions form a rapid when a river's flow is forced into a narrow channel. This pressure causes the water to react differently to riverbed events (rocks, drops, etc.).
And lastly, there is obstruction. A boulder in the middle of a river can create constriction to the flow of the river, as well as create a "drop" (over the boulder) and hydralics (where the river flows back on itself--perhaps back under the drop--often to fearful results for those caught in its grasp).
These days the term has a broader meaning applying to any river or creek that has a significant number of rapids and the term is also used to specify a kind of kayaking, canoeing or rafting on these rivers. Ex. One of his hobbies is whitewater kayaking.
Rivers in the eastern section of the United States are usually considered "technical," which means that due to lesser water volume, rafters/kayakers must often direct their craft through boulder-strewn sections of river, through narrow channels, through shoals, and so forth. This requires a degree of "river reading" skill, paddling precision, and so forth.
In the western United States, the more noted rivers (e.g., Grand Canyon) have much larger water volumes, and thus require a different set of rafting skills. Western rafters also navigate many small, low volume rivers, some with much steeper decents than eastern rivers, however, since the mountains are newer in the west, the hazard from undercut rocks, a problem in the east, is replaced by more frequent log jams created primarily by logging activities near the rivers.
The big water rivers usually do not require the precision paddling of smaller rivers, but have larger rapids and longer wilderness trips are available due to the greater length and water flow of the big rivers. The smaller rivers and creeks boated by most rafters offer many exciting one or two day trips with difficulty levels from I to VI.
In the eastern United States, there are several "must do" rivers for paddlers. For beginners, there is the Nantahala in North Carolina. It is a relatively gentle river, with the final rapid having the propensity to send paddlers in for a cold, exhilerating swim.
The next step up--and the most fun you can have before things "get scary"--is the Ocoee River just west of Cleveland, Tennessee. The 1996 Olympic Kayak Competition was held on this world-class river. A special section was contructed for the venue, but the "lower" Ocoee is the classic--and best--ride. What makes this river special is that it is almost continuous whitewater. Many river are "drop-pool" rivers, with rapids far apart. Not the Ocoee. Within ten seconds of your first paddle stroke, you will be in a serious class IV rapid. While there are some challenging rapids on the Ocoee, most of the rapids are very forgiving. If you fall in, you'll get a good scare, but will very likely be pulled back into the boat within thirty seconds.
The New River in West Virginia is the next step up. It is a "grown-up" Ocoee river. Larger rapids, fairly close together (except for the long wait for the first one--"Surprise").
The Chattooga (Sections 3 and 4) outside of Clayton, Georgia can, at high water, be thrilling to the point of fear. Huge rapids, big drops, and thunderous power, this river is a challenge for even experts.
The Gauley River in West Virginia, especially at the "Fall Draw Down" (when the reservoir is drained) is a world-class ride. Huge rapids, many of the listed as "class V+" (which is shorthand used for insurance purposes). A true challenge is to do the upper and lower Gauley in a single day. It's twenty-four miles of big rapids and lots of paddling.
Then there is the river that only the most hardcore rafters and kayakers of the east know about or raft: The Russell Fork Gorge. Located in the Breaks Interstate Park on the border of Kentucky and Virginia, this river drops 150 ft. per mile in the gorge. Huge and fearful drops. Advanced paddlers only. El Horrendo is the showcase rapid due to its size. The best description of that rapid would be "something like a 45 degree waterfall."
In the west, most rafters start on the American and work their way up to the Rogue, the Illonois, the Toulomne, the Salmon, the Snake, and then the big water rivers like the Green and Colorado through the Grand Canyon, or the Fraser in British Columbia, and many Alaskan streams where you will get tired of eating fresh salmon and have to carry a cannon to keep the bears away.