I was wondering if someone could either give me a quick run down or point me in the direction of a web-page that gives a run down of the grades of bike components for road bikes. (Sora, Tiagra, 105, Ultegra, & Dura-Ace)

What makes one so much better then another making one bike cost $300 and another cost $1000? Which parts are just worth the upgrade if you can afford it and which are just icing on the cake? I know it's all opinion or relative and there is not one right answer, but I was just looking for opinions. Its a bit overwhelming to someone who's just ridden a MTB tank. (Specialized HardRock)

I need to invest in a road bike and while I don't need a top of the line one I am doing Triathalon training so I need something halfway decent. I rather invest in a road bike that is more multi-purpose then a Tri bike given I'm only starting out in Tri's this year.

It's a loaded question I know so even if you could just point me in the right direction where to do some research I'd be ever so thankful.