Espresso shots are a result of hot water being forced through a bed of ground coffee. Ristretto shots, also known as short shots, are exacted using half the amount of water as a regular espresso coffee so they also result in about half the volume. Since they are more concentrated that regular shots they are darker and have a stronger and less bitter taste. Lungo shots, or long shots, use about twice the amount of water than a regular shot and result in twice the volume. They have a more diluted taste and tend to be more acidic.
Just because long shots are diluted doesn’t mean they aren’t good. Sometimes the acidity of the long shot can benefit certain drinks. For example, I prefer brewed coffee over Americanos, but when I do get an Americano I ask for long shots as the bitterness reminds me of my favorite roast: blonde. However, I do prefer ristretto shots over both regular and long shots most of the time. The richness of a ristretto shot goes perfect with hot, creamy milk. The stronger taste also keeps the espresso drink from being overpowered by the milk or any syrups. You should generally ask for an extra shot when you order ristretto though; half the volume means half the caffeine.