A preferred stock that may be converted into common stock or another class of preferred stock, either voluntarily or mandatory at an agreed conversion ratio. 
Other provisions may include dividends, liquidation preferences, voting or management rights, and anti-dilution.
Recommended general references are "Venture Deals: Be Smarter Than Your Lawyer and Venture Capitalist" and "The Business of Venture Capital: The Art of Raising a Fund, Structuring Investments, Portfolio Management, and Exits"