Eminent domain is the power of a government to take private property and use it for public purposes. Here in the states, the power of eminent domain is recognized in the
United States Constitution, which prohibits the taking of private property "without just
compensation." Although the power of eminent domain has been traditionally used to acquire land, the government has extended the law to acquire other forms of property. The federal government procedure is established in something called the Federal Rules of Civil Procedure.
To complicate matters, each state in America has their own statute or statutes that establish how the power of eminent domain is exercised. Naturally these statutes will vary from state to state. For example, in some states, the government is required to negotiate with the property owner before instituting eminent domain proceedings. In others, the government may institute the proceedings without prior notice to the property owner.
The federal constitutional provision that recognizes the power of eminent domain implies the requirement that the property being taken must be taken for "public use". This has been broadly interpreted to include uses which are generally beneficial to the public. "Public use" includes traditional government activities such as the building of roads, the construction of government buildings, parks, and more generally beneficial activities such as protection of scenic areas, wetlands, and historic landmarks. Under really broad interpretation, the government taking of property for redevelopment by private parties has been upheld.
On to the subject of "just compensation."
The standard for "just compensation" is usually the fair market value that the owner of the property would receive if the property were being sold in an ordinary transaction. Here's where it gets a little sticky. If the government and the owner cannot agree on the fair market value, a court proceeding including testimony of "expert" witnesses is often required. There have been many cases where property owners disagree with the decisions of the court on the determination of fair market value and have had to be physically removed from their property, sometimes at gunpoint.
Last but not least, under federal law, if the property is being condemned for a project using federal funds, the state or the federal government must also pay moving and relocation expenses to the displaced owners and provide relocation and other services to the property owners and tenants.