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 used more than once.
Most 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 reusable (the space shuttle -- see the shuttle article for a discussion of its economics). Most satellites are launched using expendable launchers because they are perceived as a having a low risk of mission failure, a short time to launch, a relatively low cost, and, most importantly, no other options exist.
Many see it as unfortunate that most "modern" 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 most uneconomical launch vehicle in history (perhaps following the Space Shuttle?).
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2 Planned expendable launchers
3 Historical expendable launchers