A Pokemon has 'base stats' of HP, ATK, DEF, SP.ATK, SP.DEF, and SPD. These base stats affect how high/low these stats will grow (e.g. a Pokemon with base 50 speed will be slower then a Pokemon with base 75 speed).

A Pokemons defense is its base defense, but has factors affecting it such as IV's (Individual values), and EV's (Effort Values).

If you have read up on IV's and EV's (Although to some it can be quite confusing!) then you can see that base stats are a rough guidline of their speed which can be compared to other Pokemon, yet must have the factors of EV's, IV's and/or nature considered.

Hope this helps, unfortunately unable to give links as it is a wide topic.