Unlike thermoplastics, thermosetting plastics cannot be remoulded or reshaped. It assumes a permanent hard shape once heated.
Initially, this is a liquid or a soft solid. Chemical reactions take place when it is heated and the long polymer chains crosslink to form a three dimensional structure. You might have bright coloured melamine dinnerware at home. Melamine is a thermosetting plastic. Bakelite, which is used for insulating electric wires, also belongs to this category.
Thermoplastics can be remelted quickly, but thermosetting plastics can withstand high temperatures. It cannot be recycled for the same reason.
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