Reverb is used to give vocals a rich sound with more presence. It can also be utilized to make the vocalist sound like he's in a bigger or smaller space, as well as change the way his voice echoes. Reverb can also work with or compensate for the acoustic properties of a venue by making the sound that's reflected off surfaces sound more natural. The same reverb effect can be used on vocals and instruments or separate reverb settings can be used for each.
Pitch correction is used to smooth out vocal flaws by analyzing the waveforms of the voice and deciding what notes should come next musically. Pitch correcting programs modulate the signal of the singer's voice so that the affected sound is in tune. Pitch correction can be used live in real-time and also used for extreme pitch modulation such as creating exaggerated highs and lows or a robotic-sounding voice.
Harmony effects programs can be used to add harmony to the parts being sung. These programs analyze the notes as they're sung and create harmonic notes that will work melodically with the vocals. Harmony can be created live by the use of an effects pedal connected to a guitar. In this type of program, the guitar chords and notes are analyzed, in addition to the lead vocal, to create accurate harmonies.
Doubling is used to layer vocals and make them sound thicker. This is a common practice in recording, where singers and rappers can layer or overdub their vocals multiple times to get a stronger sound. This practice is achieved live by having vocalists perform over backing vocals. It can also be achieved through a doubling or chorus effect program. A doubling effect can be made to sound natural or more affected.