The typical guitarist will play the guitar using only downstrokes or upstrokes. This is perfectly normal, and an acceptable way of playing. Virtuoso players also know how to quickly alternate the stroke so that they are using a mixture of down- and upstrokes. This technique allows players to perform extremely quickly, since they don't have to wait for their arm to return to the original starting position. To perform this accurately, you should use the tip of the pick to increase your speed.
String skipping produces a sound that utilizes the individual qualities of each string to play a melody over a wide interval range quickly. For instance, the lowest and thickest strings on the guitar have a darker quality, while the higher strings have a light and thin quality. By skipping between strings, the guitarist gains the characteristics of each string in a single melody and greatly increases the distance between pitches. This technique creates large melodic leaps in the melody, since moving from one string to the next creates a significant change in pitch. Without the string-skipping technique, the guitarist would have to jump from one end of a string to the other.
Sweep picking is very similar to strumming on the guitar, but it allows the guitarists to play sections extremely quickly. The technique requires the guitarist to be able to use both hands equally well. Unlike strumming, with sweep picking, you want to make each note clearly heard. In a strumming technique, one hand will hold down the strummed pitches firmly, creating a blurred effect. With sweep picking, you still hold down the pitches, but you must immediately let go of each pitch when articulated. This creates the independence of each pitch and allows all of the notes to sound clearly.
Economy picking uses a mixture of alternate and sweep picking to play even faster than with either technique by itself. With alternate picking, the hands are moving up and down regardless of the string. Economy picking uses the style of picking that is most useful for the particular section played. For instance, guitarists use the alternate picking technique as long as the music stays on one string. The moment they switch to another string, they switch to sweep picking, then continue with alternate picking.