I'm Confused

I buy a lot of The Boy's clothes online. It just doesn't make much sense to buy new when he out grows them in a couple months, eBay and Craigslist have been very good to us for clothes.

But it was so much easier when he was little. Until he hit the two year mark everything was sized by month. Simple! Aside from small size variations, we had a decent idea of how big any piece of clothing was.

Then he became a "T". 2T, 3T, 4T... First off, what does "T" stand for anyways? And even more confusing, what is the difference between 4T and Sz 4? And how small is a "Small"?

Shoes are even worse. A size 8 may or may not fit depending on whether it is a toddler 8 or kids 8. I might understand if shoe sizes were indicated with a T or K, but all I usually see on a listing is the size. WTF?

Any help on clarifying this would be most appreciated.