Basically, the word "dinner" is the word used for the biggest meal of the day. For younger people in the United States, that is the last meal at the end of the night. For most older people, "dinner" is in the middle of the day. The words "lunch" and "supper" are used to refer to the meal that is not "dinner". Therefore, generally speaking, older people have breakfast, dinner, and supper. Younger people have breakfast, lunch, and dinner. I hope this helps! Let me know if you have more questions. ~ Mrs. P. (Kayla)

As you have only had answers from the states I will add a British perspective. The words used for meals in the UK correlate with social class. Breakfast is called breakfast by everybody. The midday meal is dinner for the working class and lunch for everyone else. The evening meal is tea for the working class and either dinner or supper for the rest, supper tending to be used by upper middle class and above. Class distinctions in the UK are more fluid than previously and I would say that what I have just explained is less true now than before. I grew up in the 70s, in a working class family eating breakfast, dinner and tea. Now I've become middle class I eat breakfast, lunch and dinner. Only my classiest friends eat supper.

Where I come from, in the US, they mean the same thing. :)

The answer for this depends on your generation. For young folk, "supper" and "dinner" are basically synonyms. However, "dinner" is used much more often. For older folk (60+ typically) "supper" refers to the night-time meal and "dinner" refers to a lunch-time meal. This is rather rare though and most people will use "dinner" to refer to the night-time meal as well.

Thanks you all for your answers!