No other effect is as natural as reverb. Every sound reflects off surfaces and comes back to the listener. The time it takes to do this creates a feeling of aural space. A great concert hall is made specifically to control the reflections and provide a rich warm reverberation. Electronic devices, hardware, and software try to emulate this sound. In recording, it's one of the most commonly used effects, and almost universally necessary for some instruments, especially voice. When mixing, reverb has the effect of bringing certain sounds to the foreground or background of the mix in addition to creating a natural ambience.
Reverb is a really fast echo. However, there are many different types of reverb today. The types fall into two categories: room emulations and “old school,” like plate and spring reverb. Room emulations try to re-create how the sound reverberates in different size rooms. The larger the room, the larger the natural reverb you'll get. You typically see “small room,” “medium room,” and “hall” as popular reverb choices. In the early days of recording, reverb was simulated by sending the audio either through a spring or on a large plate of metal to simulate the sound of reverb. “Spring” and “Plate” reverb have their own distinctive sounds and are now emulated by modern reverb processors and plug-ins.