The section of the IRS code which deals with
tax-free exchanges of certain property is Section 1031.
The general rules for
tax-free exchanges are that the properties must be exchanged, similar, and used
as an investment or for business.
A 1031 exchange is also called a like-kind
exchange.
Read 1031 exchange blog post on Positive Real Estate Professionals
See IRS website for more information:
Like-Kind Exchanges - Real Estate Tax Tips.