Prestressed concrete features steel cables or tendons that have been stretched so they pull inwards on the concrete and compress it. When the concrete comes under a tensile stress like the force of gravity on a concrete beam, for example, the compression induced by the steel tendons helps hold the beam together against the tensile stress. This is much like the way you can carry a stack of books held horizontal by applying pressure to both ends.
Prestressed concrete is more expensive than traditional building materials. It's even more costly than other kinds of reinforced concrete; not only are additional materials involved but the extra equipment needed to stretch the steel before pouring the concrete adds to the cost. Using prestressed concrete in a case where additional tensile strength is unnecessary -- in a concrete floor at ground level, for example -- would raise the project costs without conferring any real benefit.
When pouring concrete, you use forms to ensure the concrete adopts the right shape as it hardens. Prestressed concrete requires more complex formwork, so it has less design flexibility than other kinds of reinforced concrete, which often makes design more challenging. Moreover, the margin for error in preparing prestressed concrete is much smaller than other more conventional materials, so more care and caution must be exercised in construction.
Lifting precast metal members into place usually requires large cranes; these also add to the cost of construction. With all of these disadvantages in mind, however, it's important also to note that prestressed concrete has some important benefits. Its superior strength in tension permits engineers to design longer unsupported spans. Ultimately, the choice between prestressed and conventional reinforced concrete should be made based on the type of project and the properties it requires.