An expendable launch system is a single-use launch vehicle usually used to launch a payload into space. This is in opposition to a reusable launch system where a single launch vehicle is launched more than once.
Most orbital expendable launchers are derivatives of 1950s-era ballistic missiles. Since the entire vehicle is discarded after launch, this may seem like an expensive launch method, but in practice they are cheaper than the one currently-existing partly reusable launch vehicle (the space shuttle -- see the shuttle article for a discussion of its economics). Most satellites are launched using expendable launchers; they are perceived as having a low risk of mission failure, a short time to launch and a relatively low cost. The Magellan probe was the first planetary spacecraft launched by a space shuttle.
Many see it as unfortunate that most "modern" orbital expendables are derived from ballistic missiles, as these missiles were built to Cold War specs and with Cold War budgets, and argue that this makes for horrendously expensive launch vehicles. A prime example of this is the Titan IV, probably the costliest launch vehicle in history (perhaps following the Space Shuttle).
Expendable launch systems typically consist of stages which are discarded one by one, in order not to have to carry and accelerate parts that are no longer needed, see Staging and Multistage rocket, as opposed to the as-yet theoretical single stage to orbit system. However, there is no reason in principle why an expendable single stage to orbit system would be impossible; it is just generally more efficient to use staging when the system is not meant to be recovered intact.
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