SPDRs (Spiders) are index ETF shares that track the S&P 500, or could refer to other similar ETFs tracking other indices.
The SPDR is the longest standing ETF (exchange traded fund), and has existed since 1993. Unlike index mutual funds that track the S&P, ETFs can trade intraday, can be sold short, and bought on margin.
There are other SPDR ETFs that are spin-offs, and using “SPDRs” in the plural might refer to these as well. SPDRs are managed by State Street Global Advisors, and the S&P 500 SPDR is listed on the NYSE under the ticker symbol SPY.
Some SPDRs are actively managed or a hybrid of active/passive, but most are passively tracking their respective index.