Tax implications Selling then buying

[Derek G.]

Hi All!

I know this question has probably been asked a million times. However, my searches are proving futile. I apologize in advance. I have currently ony purchased houses, but am now looking at selling some of them.
1. I sell a house for $150,000 (it's paid for)

2. Buy a house for $100,000

Do I owe taxes on the proceeds from the $150K deal, or do I owe taxes on the $50K left over after the new purchase? 
This is a major oversimplification, but it's just to satisfy my curiosity.
Thanks in advance guys/gals