Well, you have found the answer from a true creampie fanatic. While there are loads of definitions on the internet, this is my opinion.
A creampie is what happens after the recepient (male or female) gets semen splattered "onto" the ass or pussy. Sometimes a creampie is often confused with an internal cumshot! These are completely different.
The internal cumshot was derived from the word creampie. An internal cumshot is when the recepient actually receives the cum load "internally", inside the ass or pussy, hence the word "internal". Lovers of creampie usually like internal cumshots better because they like to watch the semen ooze out of the hole it was deposited in. People that participate in this sexual act usually like to eat it. Hence the phrase "eating creampie" or "creampie eating".