As the name suggests, hall reverbs mimic the properties of sound produced in large rooms such as concert halls and cathedrals. This type of reverb usually sounds very three-dimensional and adds a pronounced sense of space and distance to the original sound. Decay times are typically longer than other reverb types, and the overuse of hall reverb can make a recording sound jumbled or washed out.
Similar to hall reverb, room reverb mimics the properties of sound produced in smaller rooms, where the walls are closer together and sound doesn't have as much distance to travel. As such, room reverbs generally decay faster and add less of a sense of space and distance. Room reverbs are great for adding a sense of realism to instruments close-miked in small rooms. For example, adding a bit of room reverb to a dry guitar signal can help it sound as if it were recorded in a live room. This helps the listener feel like he's actually listening to a guitar played through an amplifier on-stage, as opposed to a small recording booth. When used on vocals, room reverb can add ambience without unwanted echo.
In the 1960s, audio engineers learned how to create reverb effects by hanging a large plate of metal from a frame using spring clips and then applying an audio signal to one end of the plate. The input signal causes the plate to vibrate and the change in sound is picked up by contact microphones attached to the opposite end of the plate. Plate reverb is closer to the real sound of a large room than spring reverb, and plate reverb units are still popular with many engineers. However, they're usually very large and heavy, making plate settings popular in processors. Plate reverb is often used to make a guitar or vocal track sound bigger and more present without adding a sense of distance to the sound.
Spring reverbs are most commonly found inside guitar amplifiers. As the name implies, an audio signal is passed through a series of coiled springs, which become excited and vibrate. This vibration creates a reverb effect that doesn't necessarily replicate any natural sound, but rather imparts a bouncy or twangy effect that works particularly well with guitars. External spring reverb units are also available, but again, these are designed for use with guitars and may not be ideal for other purposes. Spring reverbs found inside processors can be used to impart a low-fidelity sound to vocals and other instruments, or provide a unique vintage effect for instruments such as pianos.