If you buy a professional quality tripod – and if you're serious about your photography why would you consider any other kind? – it will usually come without a head, which you'll have to buy separately. This is because there are several different kinds of head available, suitable for different tasks and types of camera. You may also want to buy your tripod from one manufacturer and the head from another, which you can do thanks to one of the few instances of true inter-brand standardisation, the 3/8th-16 head mount thread.
If you have a good quality film or digital SLR or a medium format camera you'll need a fairly heavy duty head, especially if you often use large zoom or telephoto lenses. Traditionally supporting a heavy camera has always meant using a three-axis pan-and-tilt head, but these are difficult to adjust quickly which is a problem if you're trying to pan and tilt simultaneously, for example to track a bird taking flight. However modern materials have led to the development of good heavy-duty ball-and-socket heads, such as the latest and most advanced product from Manfrotto, the 468MG Hydrostatic Ball Head.
The 468MG is designed to support large SLR or medium format cameras fitted with heavy telephoto lenses, and can handle a weight of up to 16Kg, more than enough for even the heaviest DSLR. The Canon EOS 1D Mk IV weighs just under 1.2Kg, and even a big professional telephoto lens such as the £10,000 EF 800mm f/5.6 L IS USM only puts the total weight up to 5.7Kg, well within the 468MG's specification.