Private equity is investment in companies that are not listed on a public exchange — buyouts of mature businesses, venture capital for startups, growth equity for companies between the two. The defining differences from public-market investing are not the underlying businesses (companies look much the same on either side of the listing) but the access restrictions and the liquidity terms.
Access is gated to accredited investors and qualified purchasers, two SEC categories with progressively higher net-worth or income thresholds ($1M and $5M respectively as of writing; see Regulation D Rule 501). Fund minimums typically run from $250,000 to several million per commitment, and capital is locked up for 7–10 years — you commit, get called over years one through four or five, and receive distributions starting around year five through wind-down.
Private equity covers a wide spectrum of strategies, fund economics (the 2-and-20 fee structure and its variants), tax treatment of carried interest, and the realistic question of whether the median fund justifies its fees. All of that is in chapter “Alternative Investments”.