General Instrumental Techniques
Falls - Falls are when you play a note, and there is a fall written next to it, you fall off it by sliding downwards in pitch.
Glissando - Glissando is where you glide from one pitch to another. This could be downwards or upwards.
Appoggiatura - If there is a appoggiatura, play the grace note quickly with emphasis on that note before playing the main note.
Acciaccatura - Similar to appoggiatures, acciaccaturas are the exact same but you place emphasis on the main note.
Trills - A trill is where you alternate between two notes very quickly, until the next written note or until it says stop.
Guitar Specific Techniques
Hammer-on - A hammer-on is when you pick one note and then hammer a second finger down onto the same string to get a second note.
Pull-off - A pull-off is a hammer-on in reverse, but when you take your other finger off the fret, you pull on that string a little.
Slides - A slide is similar to a glissando, but when you move to that note you do not play it again.
Bend - A bend is where you push the string out of the normal alignment and then bring it back again.
Pre-bend - Similar to a bend, you bend the string before playing the note to raise the pitch.
Instrumental techniques are very useful because without them, music would be boring as every note would be played the same.