A landowner can give the public the right to use a route over their land, that is, 'dedicating' the route as a public right of way. Legally a dedication must be for all the public, and for all time, but the landowner can determine who can use the route, for example pedestrians only, or pedestrians, cyclists and horse riders.
Sometimes routes are used as public rights of way over time. The presumption here is that, at some time in the past, the landowner either dedicated the way as public, but has lost the evidence of the dedication, or the landowner made no objection to the public using the route. To be legally acceptable, the public must have been openly using the route, without interruption, for a period of twenty years.
Landowners can deposit maps with the council showing any public rights of way on their land, together with a declaration to the effect that they do not intend to dedicate any other routes. Hampshire County Council has more information on this.