What is this?

I've read about it here, but am unsure about exactly what it is.

It's a program you can use for free online (unless you purchase the pc version) where you enter in what you've eaten and how much and it calculates your calories, proteins, fats, carbs, etc. It's how we keep track of how many calories we've eaten

Along with what LLV said, it also tracks your exercise, and goal progress. Even though I just started using it yesterday, I like the fact that it's not restrictive, and you can add your custom foods. There's tons of pre-set foods already there, but I prefer creating my custom ones so I can grab the nutrition label from what I'm eating and directly enter it so I know I'm not cheating myself by under or over estimating. You can check mine out by clicking the lick in the light gray box on the left below my avatar.

I'll throw in my 2 cents for the version I use called nutridiary.com. The two sites are similar but each has some unique features. I hav learned a LOT by logging my food!