They are the most heavily traded futures on earth. A fund that wants market exposure tomorrow morning buys index futures rather than a hundred stocks; a fund that wants to reduce exposure sells them rather than unwinding positions it intends to keep.
All of them settle in cash against a special-opening-quotation, list quarterly months (H, M, U, Z), and trade nearly 24 hours on globex. Because they are cash settled there is no delivery risk, so limits are set as accountability-level rather than hard caps, and the overnight session is where most overseas reaction to US news is expressed.
The family runs from the flagship es and nq down through ym and rty to the micro-futures versions at one-tenth the size, which put index exposure within reach of a four-figure account.
Example: hedging a $2,000,000 portfolio with a beta of 1.0 at an index level of 5,000 needs 2,000,000 / (5,000 x 50) = 8 ES contracts, or 80 MES if finer sizing is wanted.
Related: es, nq, ym, rty, futures-fair-value