Kirk Sorensen reports that the short answer to the title question is that uranium-fueled reactors can be built right away, but they use fuel inefficiently. Thorium-fueled reactors, on the other hand, are fuel-efficient, almost perfectly so, but that comes at the end of a three-phase process, with the first phase shared by thorium and uranium fuel.
Uranium is the only element with a naturally occurring fissile isotope, meaning the isotope nucleus splits into two smaller nuclei when struck by a neutron traveling at any speed. Few isotopes are fissile. But all thorium and nearly all uranium is fertile, meaning that when they absorb a neutron, they undergo a double-beta decay and become fissile materials.
This has both positive and negative implications. Nearly 100% of natural uranium is the isotope U-238 (99.3% to be exact), so its behavior in a reactor dominates the discussion of nuclear energy. Except in rare circumstances, U-238 will fission when it absorbs a neutron, but it does decay on its own over a few days into Pu-239.
Essentially all natural thorium is the isotope Th-232, and it also does not fission when struck by a neutron (except under rare circumstances). After absorbing a neutron, it decays in about a month to U-233.
U-233 is special in that it will likely fission when struck by a neutron of any energy and release enough neutrons to continue converting thorium into new U-233. It acts a bit like a nuclear catalyst, letting thorium be consumed like a fuel in properly designed thorium reactors.
If a thorium reactor is started with U-233 and it is frugal with its neutrons, the reactor can use thorium as a fuel and release essentially all of its energy. This is the main reason people have gotten excited about thorium reactors. Full energy release means no wasted fuel at the end, which eliminates the problem of long-lived nuclear waste.
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