An employee stock ownership plan, or ESOP, is a trust established by a corporate which acts as a tax-qualified, defined-contribution retirement plan by making the corporation's employees partial owners. contributions are made by the sponsoring employer, and can grow tax-deferred, just as with an IRA or 401(k). Unlike other retirement plans, the contributions must be invested in the company's stock.[1]
[1] From InvestorWords.com, http://www.investorwords.com/1749/ESOP.html