Explaining modern Pokémon games

[SERIOUSLY THOUGH]

So here's the thing - When I was younger, I played Pokémon for hours and hours on end. Completed Blue, and Gold. Then I played Diamond, and completed that, mildly confused as to why my Pidgey had to enter a beauty contest.

I've followed Pokémon from afar - shaking my head at ice-cream Pokémon whilst vehemently denying the fact that having a rat Pokémon called Rattata was pretty much the same level of originality - though haven't actually played a proper game in a long time.

What I'd like is a brief overview of changes that have occured over the times.
e.g. What the devil is IV/EV, nature, abilities, and so on and so forth ...?

Also, what do you dis/like about the changes ?

[spoiler]I will reverse-double-supersonic-ancestral-brunch-bitchslap anyone who says something like "omgzz00rdz 1st Gen wuz de Gr8T5t", "4.5st g3n iz fur babeiz", or "Do you even lift"[/spoiler]