Holodecks & Holosuites
Room on a starship, in which holographic simulations are created for investigation and entertainment. Starships have main holodecks and several small ones, holosuites, suitable for personal use.

A holodeck can create simulations in the following ways:
=/\= The walls can generate holographic images which appear to extend for an unlimited distance. 
=/\= Holograms can be projected into space.
=/\= Holograms can be augmented with force beams to simulate solid, tangible objects.
=/\= Holograms and force beams can be augmented with replicator technology to provide actual substance. Food on the holodeck would be of this nature. Most inanimate objects which are likely to be eaten or have properties difficult to simulate (e.g. water) would be simulated in this way.
=/\= An animate object is comprised of a partially stable form of matter created by the Holodeck replicators for use in the holodeck only. This material is stable only within a holodeck or holosuite, and degrades into energy if removed. This is matter held in place by force beams operating at a molecular level, as opposed to actually replicating the object down to the molecular bonds. Without the force beams, the object disintegrates.

=/\= Note that the wall/hologram/force beam simulations are easy for the holodeck to carry out. Also, if an object's status changes the simulation type will change seamlessly as well. For example, someone sees a distant tree (walls), approaches it (hologram), leans on it (force beams), breaks off a branch (holodeck-matter), then picks and eats an apple (replicated). But how can people walk for hours and hours without bumping against the walls? The Holodeck has a force field treadmill. If its occupants get too close to the walls, they are shifted away. Since the Holodeck can modify its gravity in 3 dimensions, the occupants won't notice any inertial change.