A thermoset is a kind of
polymer (that's
plastic to you, folks) that forms a threedimensional
network of polymer chains during
polymerization. Unlike
thermoplasts and
elastomers (
rubbers), thermosets do not
melt or
soften upon heating, but rather
burn and/or
decompose.
Thermosets (mostly
polyester) are often used in combination with glass fibres, to make things like
boats and
airplane wings. This combination of
fibre and polymer is almost as strong as
steel, but much lighter.