Guitar alternate picking is used by guitarists like Paul Gilbert, John Petrucci, Al Di Meola, Steve Morse, Zak Wylde and more. Some guitarists like Al Di Meola and Steve Morse use it almost excluseivley. So what is alternate picking? It refers to specific method of picking from guitar string to string with the use of a pick.

Alternate picking is generally done by default when picking a single string, that is the pick changes direction from downward to upward each time the single string is repeatedly picked or vice versa. Sometimes a guitar player will play on a single string with repeated consecutive downstrokes or upstrokes in a row, for example on a power chord in a metal song like you'll see with Metallica. This is not generally the way to play repeated notes on a single string except for some specific styles or rhythm effect. With alternate picking you keep the pick alternating down up down up always changing direction when moving from string to string. Advocates of alternate picking believe that it leads to more control over tone and consistency with timing when playing.

So really where you will come across the concept of alternate picking is in regards to a very specific and prescribed method of moving across the guitar fretboard from string to string.

Whether or not a player uses alternate picking exclusively or not as their picking method, generally he or she will also use other techniques as well.