Stats
Every Pokémon has stats in HP, Attack, Defense, Special Attack, Special Defense, and Speed. All Pokémon have different Base Stats in each stat too, and these determine how good a stat will be on a Pokémon. For example, a Kingdra has a base stat total of 540, and has different base stats in each stat. Kingdra's Attack stat is 95, for example. This does not mean, however, that Kingdra will have 95 Attack, as base stats are more of just a way to compare stats in Pokémon and place them in a Tier List. A Pokémon's base stats influence what kind of stats it will have at any level, for example, Kingdra has a lower base Speed than Attack, Defense, Special Attack, and Special Defense, so its Speed will likely be lower than all of those stats. This will be true for most Pokémon, but not always in all occasions.
Also to note, in Red, Blue and Yellow, there is no split between Special Attack and Special Defense. It's simply called "Special" in these games, which means that some Pokémon, such as Hypno and Cloyster's Special is very high in Red, Blue and Yellow, but in Gold and Silver onwards they have lower Special Attack and Special Defense, respectively. This was changed in Gold and Silver.
For all the Pokémon and their base stats, check out Bulbapedia's Base Stats Page.
