According to "Lerner et al. (2004) and Gompers and Lerner (2002)" Private Equity funds are typically structured as limited liability partnerships in which a specialized Private Equity firm serves as the general partner (GP) and institutional investors or high-net-worth individuals provide the majority of capital as limited partners (LP). The GP undertakes investments of various types (e.g. venture capital, bridge financing, expansion capital, leveraged buyouts), with the obligation to liquidate all investments and return the proceeds to the investors by the end of the fund's life.
Summary of private equity characteristics:
High risk asset class As private equity market is not transparent, as there is little publicly available information. So investing in private equity is riskier than investing in other publicly traded firms. The risks and illiquidity of investments are compensated for by a higher expected return.  Long-term time horizon An engagement in private equity is usually long term investment with usual life span of 10-12 years. Illiquid asset class It is generally structured as a fixed limited partnership and often there are sale restrictions. Hence it is generally regarded as illiquid.
"Today most private equity investments are also financed by debt. The use of debt in addition to equity is known as "gearing" or "leverage" and is common in the financing of most companies. Private equity companies are highly geared as compared to other publicly traded or private companies". Private equity firms are very much aware of the advantages and risks of using debt and have been used it as an integral part of their competitive advantage as buyers and owners of a business. Debt is often repaid from the cash flows of the portfolio company, thus enhancing the value of the equity. Mature companies with stable cash flows are able to support a higher degree of borrowing, allowing repayments to the equity investors as compared to newer companies. Levels and types of debt tend to reflect the prevailing interest rate climate and the opportunities for acquisitions and internal investment available to the company .
 BVCA's "Guide to private equity" by Keith Arundale
 Matthias Huss, October 2005 "Performance Characteristics of Private Equity"  Matt Krantz, USA Today " Private Equity Firms Spin Off Cash"  House of Commons, Treasury Committee - Private equity -Tenth Report of Session (200607),Volume I Private equity firms help out companies to secure capital for expansion and consolidation. With high levels of private equity debt, firms taken over by private equity groups are prone to interest rate hedging to levels that may, in some unfortunate instances, become an unsustainable drain on the resources of those firms. But when the debt levels in the capital injected grow to levels that may not be sustainable over the longer term, high interest rates can cause debt levels to topple a company. An expected turnaround can be crippled by a slowdown in the economy. So...