An easement basically means that someone else has the right to do something on your land without your permission , ie lay gas, water or utilities for themselves. Also owners of land that an easement is on can prohibit them from doing certain kinds of work to them,that would affect their neighbour's ie building a large wall that would block out light or laying foundations for a structure in an area of land that a neighbouring land owner has access to.

I would not disagree with the comments made regarding easements only expand on it. They are by and large in place with prior agreement on parties for a variety of reasons (rights of access over land, installed service apparatus for gas/water/telecoms/ sewers etc and their maintenance). Easements come in many different forms and they are usually particular to that site/ piece of land. The can and do often have particulars that have restriction's or limitations on what is allowed to be done on the land/ property but it is not to say they cannot be changed in the future. If unsure with the status of an easement it is always best to seek professional advice (such as solicitor)

If I buy a bit of land of someone with an agreement that I'm not going to build on it a easement is to say I have to pay them the difference between Greenland and building blot

I suggest that you have a look at the following link.

It has all of the relevant info you might need...

http://www.boundary-problems.co.uk/boundary-problems/easements.html