TM stands for Technical Machine. They're useful for teaching Pokémon moves they would not normally learn via Level Up. TMs are typically found in the wild or in Poké Marts, but can only be used once, unlike HMs which may be used repeatedly.
Generation I featured 50 TMs, some of which were removed as TMs in later generations. Generation II remained with 50 available TMs, but did introduce some new ones whilst removing others. Generation III also remained with 50 TMs, again changing the list of TMs slightly. By Generation IV, and additional 42 TMs became available, totaling at 92 TMs. Generation V is due to mark a huge impact on the function of TMs, allowing them to be used repeatedly - much like HMs, except the total number of TMs reached 95.
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