A junction box is a metal or plastic box in the middle or at the end of a conduit run. Metal junction boxes provide their own ground bonding. Plastic junction boxes often require a grounding wire to be run through it to ground the item the wire run is powering.
If the wires go through without being broken, terminated, or spliced, it is called a pull box. What makes it a junction box is the idea that the ends of wires can be found inside of it. However, this does not necessarily mean that the wire run has ended here, it could be continued with a splice, wire nut, or a terminal block -- that is, a junction in the wire.
Junction boxes and pull boxes are handy in a number of circumstances. Being larger and more spacious than the conduit, they allow easy access to the wires inside by opening the cover if they need to be worked on. A junction box might be installed next to a permanent air conditioner, for example, for ease of disconnecting the wires if it needs to be removed or replaced. They are also used to split a conduit run into several smaller runs. For example, a single pair of wires can be split off from a junction box, branching out to power several lights in a large conference room. Junction boxes can also hold light switches and electrical outlets.
To make connecting conduit to the box easier, many models have circles pre-stamped into the sides of the box, left attached by a small, easily broken tab. These disks are called knock-outs, and their removal leaves a nice clean hole to attach the conduit to. Most knock-outs are in the form of concentric circles, so conduits of various sizes can be connected by removing as many disks as necessary to form the appropriately sized hole.
Some jurisdictions, however, discourage or prohibit the use of knock-outs because they form a poor ground bond due to the small tab holding them in (check your local codes). Small conduit only requires the middle concentric circles to be removed, leaving the larger stamped circle in place around it so the conduit is only attached to the rest of the box by the comparatively delicate tab. In these applications a solid box is required, and holes must be cut in them as needed.