Easements are rights attached to property and remain with the property even after the property is sold and the title is transferred.  This type of easement is known as an easement appurtenant.  An easement appurtenant runs with the land and if the property is bought or sold, it is bought or sold with the easement in place.  The easement becomes part of the legal description of the property.
Another category of easements is easement in gross, which could not be transferred and were not tied to a particular piece of land.  For instance, a person could grant an easement across a residence to a neighbor, but this type of easement would not necessarily continue with the new neighbor if the neighbor holding the easement sold the property. These types of easements are referred to as personal easements.  However, not all personal easements are non-transferable and law permits the transfer of commercial easement, such as a utility easement.