What is a stock index (and what it's for)
A stock index (S&P 500, Nasdaq, IBEX 35) is a "basket" that summarises the behaviour of a group of stocks. It\'s the thermometer of a whole market, and reading it gives you context to trade any asset.
What an index is
An index groups a set of representative stocks and calculates a single value reflecting their combined behaviour. The S&P 500 gathers the 500 largest US companies; the Nasdaq, many techs; the IBEX 35, the 35 largest Spanish ones. When "the market rises", it usually means its benchmark index rises.
How they\'re calculated
Most major indices are weighted by capitalisation: the largest companies weigh more in the index. That\'s why, in the S&P 500, the biggest techs move the index far more than the small ones. This is key: an index can rise "thanks to" a few giant companies while most don\'t move.
What they\'re for to the trader
Indices are the market\'s background context. Their direction tells you the general sentiment (risk-on or risk-off): when the S&P 500 rises strongly, there\'s risk appetite; when it falls, fear. Trading an individual stock without looking at its index is ignoring the current you\'re swimming in.
Indices as a tradeable asset
Beyond a reference, indices can be traded directly (via CFDs, futures or ETFs). They offer exposure to "the whole market" at once, with the advantage of being diversified (you don\'t depend on a single company). They\'re among the most traded assets in the world for their liquidity and clean trends.
Where to start
Add the S&P 500 (or your market\'s index) to your daily radar, even if you trade other assets: it gives you the background sentiment. And if you\'re interested in trading indices, start with the largest, most liquid ones (S&P 500, Nasdaq), which have clearer trends than individual stocks.