This guide really helped me when I was first getting started.
http://www.gamefaqs.com/portable/ds/file/925601/48500
The section most relevant to your question:
Base stat: A base stat is a value on a pokemon that stays consistent with all
pokemon of its species. All Tauros have a Base Attack Stat of 100 and all of
them will have an attack of 205 at level 100. The only things that can change
that is IVs, EVs, and natures.
Individual Value: An individual Value (IV) is a hidden value. It is a set
random number from 0 to 31 that is added to the base stat of your pokemon. In
simplistic terms, if Tauros has a base attack of 100 and an IV of 31, it has a
total of 236 attack at level 100. Each of the six stats have their own IVs. You
cannot change the IVs of a pokemon, it is set the moment you catch it or when
you recieve the pokemon as an egg. The higher the IV, the better.
Effort Value: An effort value (EV) is a hidden number you can manipulate to
increase the stat of a pokemon by either battling pokemon or using vitamins. In
simplistic terms, if your pokemon battles pokemon with high base attack, your
pokemon will also increase its base attack. Effort values allow you to do
whatever you want to make your pokemon better in particular stats.
Nature: A nature increases one stat of a pokemon by 10% and decreases a stat of
a pokemon by 10%. For example, an "Adamant" nature increases attack by 10% and
decreases Special Attack by 10%. Some natures do not increase or decrease
stats.
While it isn't perfect, it's good enough for you to get the general idea of ivs and evs