What makes these questions classified in any of these categories?
You can hover your mouse over the tabs and it will give you a brief, one sentence summary for each.
Here's a listing of tabs groupings and their summaries,
interesting: questions that may be of interest to you based on your history and tag preferences
featured: questions with an open bounty
hot: questions with the most views, answers, and votes over the last few days
week: questions with the most views, answers, and votes this week
month: questions with the most views, answers and votes this month
newest: the most recently asked questions
faq: questions with the most links
votes: questions with the most votes
active: questions that have recent activity
unanswered: questions that have no upvoted answers