Auto-Tune began as a corrective tool, chiefly used for bringing in tune notes that were slightly sharp or flat. By setting the Auto-Tune parameters to extremes, you can create some very unusual and distinctive sound effects. The popularity of using Auto-Tune as a creative tool is typically attributed to Cher. Her hit song "Believe" features the effect prominently. The producer achieved the characteristic "wobbly" vocal sound with Auto-Tune.
This is a much more subtle effect. Producers use reverb to soften the sound of a vocal, guitar, drum or piano. Reverb approximates the sound of various acoustic environments. For example, typical reverb types include "large hall," "cave" or "church." Reverb is responsible for the eerie vocal sound on the George Michael track "Jesus To A Child." Typically, a producer will apply reverb to make the various sounds on the CD blend together, and the effect is therefore less immediately noticeable.
When a sound appears to jump right out from the speakers, it is often the result of hard panning. When mixing engineers work on tracks before release, they pan every sound so that it sits in a certain place in the audio spectrum. If you listen to the intro to the song "Dead Skin Mask" by Slayer, you can hear the effect of hard panning. This effect is especially prominent when listening through headphones. It sounds almost as if the two guitar players are standing at either side of you.
This effect is typically used to make guitars sound heavy. Unlike Auto-Tune and panning, a guitarist would typically distort his guitar with a distortion pedal while it was recorded, rather than after. There are a range of distortion types used in popular music. For example, the distinctively "fuzzy" guitar sound in the beginning of "All Day and All of the Night" by The Kinks is a natural overdrive sound from the amp.
A producer uses compression to limit the frequency range of a sound. It is typically a corrective process that stops certain frequencies from peaking in the mix. However, when the compression parameters are turned up high, it creates a distinctive "squashed" sound effect. The verse vocals on Green Day's "American Idiot" are heavily compressed, making singer Billie Joe Armstrong sound like he's singing over the telephone.