For much of its history the Forest of Dean, using the term in the particular sense of the area subject to forest law, included two distinct sorts of land: on the one hand there was land held by the Crown in demesne, mainly extraparochial and uninhabited woodland and waste, and on the other manorial and private freehold land, mainly cultivated, settled, and formed into parishes.
Extracted from A P Baggs and A R J Jurica, 'Forest of Dean: Bounds of the forest', in A History of the County of Gloucester: Volume 5, Bledisloe Hundred, St. Briavels Hundred, the Forest of Dean, ed. C R J Currie and N M Herbert (London, 1996), pp. 295-300 available online [accessed 6 February 2015].
The link above will take you to a full explanation of the complex administrative and institutional history of the area. For our purposes we will adopt the broader approach of regarding the Forest as the whole of the area between the Severn and the Wye north east of Chepstow, roughly as far as Bromsberrow.