Here is my story in a nutshell: I was in the USA for 4 years. For 2 years I was there on a J-1 visa and than I made a silly mistake and decided to stay there illegally for another 2 years. Recently I decided to come back home to Europe. At the airport I was expecting to get a stamp in my passport or at least be questioned about my illegal stay in America but none of that happened...
Now that Im back home I was thinking that I would like to visit the States and applied for the ESTA program which would basically let me visit the US for 90 days without a visa and I got APPROVED! I was so happy that maybe I would be able to visit this beautiful country legally and maybe have a chance to see my friends because I miss them dearly.
My only question is that should I have a problem at the US port of entry because of my previous visa overstay? I just don't understand how I was able to get approved for ESTA... I really do not know what to do. Should I book a ticket and try to visit the US but Im afraid I will be turned back by the immigration officer.
Any help would be appreciated and thank you for taking the time to read my post! God bless! :-)

If you overstayed period was after 9/11, it's likely that immigration officers will find your prior record... not guarantee.