The term spider is the commonly-used expression to describe the the Standard & Poor's Depository Receipt (SPDR). This type of investment vehicle is an exchange-traded fund (ETF). You can think of an ETF as a basket of securities (like a mutual fund) that trades like a stock. In the case of spiders, the basket of stocks is the S&P 500 index. One of the reasons for buying a SPDR is that it is a quick and easy way to have significant diversification. SPDRs are also relatively inexpensive compared to what it would cost to create this type of portfolio yourself.
SPDRs contain one-tenth of the S&P 500 index portfolio, which is why the cost to buy one unit of this asset is nearly equal to one-tenth of the S&P 500 index level. SPDRs trade on the American Stock Exchange (AMEX) under the symbol SPY. Like all ETFs, they trade in the same manner as regular stocks having continuous liquidity and provide regular dividend payments. This type of investment is ideal for those who believe in passive management, a strategy that attempts to mirror a market index with no desire to try and beat the market. See our Special Feature: Exchange-Traded Funds for everything you need to know about ETFs.
For more on index investing, check out Index Investing Tutorial, Introduction To Exchange-Traded Funds and Advantages Of Exchange-Traded Funds.
Spider, or SPDR, stands for Standard & Poors Depository Receipt. It is an exchange traded fund, so it is an investment vehicle just like a mutual fund. You would purchase a SPDR if you wanted to invest in whatever the SPDR is investing in--which could be US stock, non-US stock, gold, etc. SPDRs are built and managed by State Street Global Advisors. If you invest in SPDRs, you will want to assemble a diversified mix of them so that you end up with a diversified portfolio.
For example, you might consider a multi-asset portfolio that utilizes 12 mutual funds or ETFs (exchange traded funds). A SPDR is an ETF. Each fund is given an equal allocation of 8.33%. A 12-asset portfolio been “pre-built” and is available at Motif Investing for a $250 minimum investment. https://www.motifinvesting.com/motifs/7twelve-core-portfolio.
SPDR S&P 500 fund. Ticker: SPY.
It's the largest and oldest ETF and ubiquitous among traders and investors for gaining exposure to (owning a piece of) the US Equity Market.
To answer your question literally, you only need a few hundred dollars to buy one (1 share). Although, I would recommend talking to a professional about how much you might need, want, or IF you should buy any at all, to achieve your goals.