iShares, Vanguard ETFs and spiders each represent different exchange-traded fund (ETF) families. In other words, an individual company offers a range of ETF types under one product line. Because these ETF families are constructed and operated by different companies, you will find differences in terms of how they are made up and what indexes or sectors they cover.
BlackRock is the company behind the iShares family of ETFs. The company offers a large selection of more than 350 funds, which cover a wide range of both U.S. and international sectors and indexes, as well as asset classes, including bonds, real estate and commodities.
The Vanguard ETF family, formerly known as the Vanguard Index Participation Receipts (VIPERs), is similar to iShares in that it offers a wide range of ETF types covering numerous indexes and sectors in more than 50 different funds.
State Street Global Advisors' Spiders (SPDRs) are index funds that were initially based on the S&P 500 index, but branched out to include other investment options as their popularity grew. Some of the best-known spiders are the 10 Select Sector SPDRs that cover individual sectors of the S&P 500. (For more, see What are SPDR ETFs?)
The differences between spiders, Vanguard ETFs and iShares are primarily based on the companies behind these ETFs and which indexes and/or sectors they cover. But if you are looking for exposure to the S&P 500, for example, which is offered by more than one ETF company, look at the more specific attributes of the fund. The biggest thing to focus on in this case will be the fund's expense ratio (a lower expense ratio is generally more desirable), along with how well the ETF tracks the underlying index.
For more insight, see the Exchange-Traded Funds Tutorial.