An easement allows one party to use another party's land for a particular purpose.  For example, a person might have an easement that allows him to use a driveway across his neighbor's property.  In many areas, utility easements are in place to allow government agencies or utility companies to place poles, wires, pipes or cables across privately-owned land.  An easement can be created for nearly any purpose if the parties agree to do so.
Some easements creations happen in the process of platting land.  Separate legal documents are responsible for creating others.  If there is an easement on your property, there are likely to be some restrictions on how you can use the land in the easement area.  A surveyor can locate an existing easement in connection with a survey.  An attorney can provide more information on how to use the easement area.
On many residential lots, there are drainage and utility easements along the sides of the lots.  Most cities do not allow buildings to extend into these easements. Also there may be other regulations.  Local Building Inspectors can usually provide information on these regulations.
If the creation of a new easement needs to occur, it is generally necessary to have a surveyor locate the correct area. Then they will prepare a legal description of it.  An attorney can then prepare a document which the parties can sign to create the easement.