Stock market indices are tools used to measure the performance of select groups of companies and give insight into the overall market. Each index is structured differently—some focus on the largest corporations, others on specific sectors or market strategies. One index might track a small group of major companies, while another could include hundreds of diverse businesses. Indices can be weighted by price, market capitalization, or equal distribution, affecting how they respond to market changes. These benchmarks help investors evaluate trends, make informed decisions, and compare the performance of portfolios against broader markets. Whether you're following a compact index of industry leaders or a more expansive list that spans the entire economy, understanding how these tools work can deepen your grasp of financial news and investment strategy. Learning the differences among indices can guide smarter choices for retirement accounts, mutual funds, and beyond.