An easement is a real (or property) right. Being a real right, an easement is enforceable against anyone who interferes with it. An easement exists in favour of what is called a 'dominant' tenement (the land which is benefited in some way by the easement), over a 'servient' tenement (the land which is burdened by the easement). They are usually neighbouring areas of land owned or occupied by different people.
A 'positive' easement gives the owner of the dominant tenement a right to do some specified thing. A 'right of way', which is a right to go through a specific route on someone else's land, is a common form of positive easement.
A 'negative' easement restricts what the owner of the servient tenement can do. An example of a negative easement is a 'right to light'. Where this exists, the owner of the servient tenement cannot construct anything that would block the natural light reaching the dominant tenement.
To be indefeasible, an easement must be registered on the title deeds of the servient tenement. If it is not registered then a purchaser who becomes registered owner of the servient tenement will not be bound by the easement.
Note: Although an easement is a real right, it would also be possible to create a personal right to have access over another person's property. But any such right would only be enforceable between the contracting parties and might not be specifically enforced.