The Nasdaq Composite Index (Comp) was created in 1971 by the Nasdaq Stock Market, one of the world's largest stock market exchanges. As the first index to track performance of the Nasdaq market, the Nasdaq Composite Index originally included all stocks trading on the Nasdaq Stock Market. Today, the index includes over 3,700 stocks including Nasdaq Global Market and Nasdaq Capital Market listings. It is a capitalization-weighted index, which means the companies with the largest market capitalization make up the greatest portion of the total index.

The Nasdaq Composite Index tracks the performance of stocks across multiple sectors and industries, with technology stocks making up the largest portion of its stocks at 54.4 %. The next largest sector is healthcare at 15.9%, with consumer services, financials and industrials following suit. Five mega-cap companies, led by Apple Inc. (AAPL), make up the majority of the index at 40.1% as of March 2021.

The Nasdaq Composite Index is most often followed by investors as a measure of performance of the Nasdaq stock market. The index is also used as a benchmark for fund managers, to assess if their portfolio is outperforming or underperforming the index.

The Nasdaq Composite Index is a closely watched index due to the large number of technology stocks included in the index, which reflects the performance of the tech-heavy Nasdaq stock market. The index is also of interest to investors due to its relatively high volatility, which is usually caused by sudden changes in the performance of a few individual stocks.

Investors and traders can track the performance of the Nasdaq Composite Index on several sites, including Nasdaq Global Indexes, Yahoo! Finance and Google Finance, as well as on the Nasdaq website. Additionally, the performance of the index can be traded using exchange-traded funds (ETFs) and stock futures, which can be used to gain exposure to the overall performance of stocks in the Nasdaq Composite Index.

Overall, the Nasdaq Composite Index is an important index for investors interested in the performance of the Nasdaq stock market, or as a benchmark for their own portfolios as well as for traders who want to bet on the outlook of this index. Its weighting towards technology stocks and the presence of five mega companies also makes it an interesting and closely watched index.