QQQ is one of the best established and typically one of the most actively traded ETFs in the world. Often referred to as the triple Qs, it's also one of the most unusual. The product is one of a few ETFs structured as a unit investment trust. Per the rules of its index, the fund only invests in nonfinancial stocks listed on NASDAQ, and effectively ignores other sectors too, causing it to skew massively away from a broad-based large-cap portfolio. QQQ has huge tech exposure, but it is not a 'tech fund' in the pure sense either. The fund's arcane weighting rules further distance it from anything close to plain vanilla large-cap or pure-play tech coverage. The ETF is much more concentrated in its top holdings and is more volatile than our vanilla large-cap benchmark. Still, the fund has huge name recognition for the underlying index, the NASDAQ-100. In all, QQQ delivers a quirky but wildly popular mash-up of tech, growth and large-cap exposure. The fund and index are rebalanced quarterly and reconstituted annually.