My simple explanation is: A person who not only loves to eat well, but has an interest in food
where/how it is grown, prefers to buy fresh locally, when possible, wants to cook it simply but well, to bring out the best flavor of the very freshest foods.
A more detailed explanation is this one I like.
To be a foodie is not only to like food, but to be interested in it. Just as a good student will have a thirst for knowledge, a foodie wants to learn about food. A foodie will never answer the question "What are you eating" with "I don't know." There are some basic traits of being a foodie, as there are basic traits that come with all labels. Generally, you have to know what you like, why you like it, recognize why some foods are better than others and want to have good tasting food all or certainly most of the time. This doesn't mean that you can't eat flaming hot Cheetos every now and again, but it does mean that you don't fool yourself into thinking that it's a nutritionally balanced meal. Do you have to know the difference between a beefsteak tomato and an heirloom tomato? No, but you might be interested to find out what it is. Do you have to only shop at farmer's markets? No, but you still look for good, fresh produce. Are there some foods you just don't like or weird foods you like? That's ok - it doesn't make you any less of a foodie. Just like food, learn about food and, most importantly, eat food.