An easement is an enforceable right to travel over property owned by someone else. For instance, I might have the right to use my neighbor's driveway to access my lot, or the electric company might have an easement to enter my property to access overhead or underground wires. Easements usually appear in the deed of the owner of the property, but sometimes they can arise without being recorded in a deed.
How can there be an easement that is not in a deed?
Generally, an enforceable right in land must be in a written deed. In a few special circumstances, however, the facts will indicate that an easement was intended to be included, but was just omitted, or that the use of the property over decades indicates that there is an easement. For example, if a a seller owns two lots, only one of which has access to a public road, and he sells the landlocked lot to another person, the law will presume that the seller intended for the buyer to be able to travel across the seller's remaining lot to get to the road. Even if an easement is not presumed, if a person makes a path and openly uses it despite the fact that it is on someone else's land for many years (in Maryland -- 20 years), an easement can be found to arise by "prescription." This argument is very much like adverse possession or squatters rights -- if one uses land deeded to someone else against the owner's interest, and does it for long enough, this can become an enforceable right.