For questions relating to recommendation engines, collaborative filtering, and personalization. Questions tend to be algorithmic or statistical in nature.
Recommendation engines are systems that suggest (recommend) to users various items for consideration. Some of the best known real world usage occurs at many major internet sites, such as Netflix, Amazon, Google, Yahoo, Pandora, last.fm, and others.
The input of such algorithms are most commonly the past purchases of the user, the rating the user gave to other goods, the items that have been purchased together ("customer who bought this also bought..."), the browsing history, etc.
For this tag, it is recommended that users focus on mathematical or statistical clarity, as it is a particularly advanced topic and implementation can take quite a bit of computational effort.
See also wikipedia.
