IVs (Individual Values) are values that a Pokemon has, and IVs don't ever change. It's like a Pokemon's innate ability. Each stat's assigned an IV from 0 to 31 the moment it appears (wild) or the egg is received (day-care), and from there they don't change.
When breeding for IVs, you can give one of the parents a Power Item (Bracer, Belt, Lens, Band, Anklet, or Weight), and the resulting egg will always have that corresponding IV from the parent. For example, if you know your Starmie has a 31 Speed IV, you would give it the Power Anklet if you wanted to breed more; all your eggs would then be guaranteed a 31 Speed IV. A total of 3 IVs are passed down from the parents, so if you wanted to pass down multiple 31s you'd have to get a bit lucky (the Power Item gives one, and the other two would be random).
Another basic building block of team building that I'd also like to point out is that you can easily cover your weaknesses - i.e. have safe switch-ins to cover your weaknesses. For example, if your Pokemon is weak to Thunderbolt or Earthquake, a Gliscor or Claydol can safely switch into both, and - hopefully - threaten the Pokemon who fired it off.
...I sound so long-winded...