Guitar techniques (or really techniques for plucked and some bowed stringed instruments) can be divided into two categories: off the string and on the string.
Off the string techniques are techniques that are utilized mainly by the right hand such as tremolo, Bartok pizzicato, sul tasto playing, and many others.
Tremolo: guitar tremolo is a technique really borrowed from mandolin playing, it is the technique of playing a note in unmeasured rapid succession. The easiest way to achieve a sustainable tremolo is to turn the pick so it is almost perpendicular to the string and oscillate quickly with the right wrist.
Bartok pizzicato or snap pizzicato: is a technique named after composer Bela Bartok. The string is pulled upwards as it is plucked and allowed to snap back on the fingerboard as it recoils. The effect is a loud sharp sound. For obvious reasons, this technique is impractical at high pitch and/or quiet dynamic.
Sul tasto: Italian for on the fingerboard. Rather than strumming near the sound hole, the instrument is played at the fingerboard producing a lighter, flutelike timbre.
Sul ponticello: Italian for on the bridge. The instrument is played near the bridge, this produces a more brittle, metallic timbre.
Muting: the right wrist rests slightly on the strings while strumming, dampening the sound.
Playing on the wrong side of the string: a new technique which can be heard in George Crumb's Black Angels the string is plucked to the left of the stopped string. The timbre is lighter but much quieter.
Tapping: a technique mostly for electric guitar, which can be heard most famously in Van Halen's Eruption. Rather than plucking the strings, the left and/or right hand taps the string at the desired fret. When the left and right hand are used in combination, one hand may pluck the first note while the other hammers another note for a trill or arpeggio.
Fingernail pizzicato: the string is pulled up with the fingernail and then released.
Arpeggiated chords: a chord is played one note at a time, unmeasured, in an ascending or descending order.
Scordatura: alternate tunings of strings. Effective to alter the range and timbre of the instrument or facilitate playing certain pieces.
Marcato: Italian for hammer. Instead of plucking the string, the right finger taps it to produce a different timbre.
Strumming techniques are off the string techniques involved with strumming the guitar. The include palm mutes, clawhammer, finger-picking, etc.
Palm mute: the right palm is thrust at the strings, stopping them short. Provides rhythmic breaks in a strum pattern.
Clawhammer: a style of strumming where the bass note is picked with the thumb and then the remaining strings are strum with the fingernails. Common in banjo picking and bluegrass music.
Finger-picking: rather than strumming or playing with a pick, the fingers of the right hand pluck different strings.
Snap strumming: the fingers are held back by the thumb or palm and snapped at the string either from index finger to pinky or pinky to index. Common in Spanish guitar and Flamenco.
Percussive knock: the guitar is struck on the body producing a percussive knocking sound. This can be done with a closed fist or by snap strumming.
On the string techniques are usually done with the left hand. They include vibrato, trills, and harmonics to name a few.
Vibrato: the finger which stops the string is vibrated and the pitch oscillates slightly. Very useful in making a line sound more expressive.
Slur: instead of plucking each note individually, the left hand pulls off the stopped fret to a lower pitch or hammers on to a higher one. Slurs are very useful in quick passages where continuous plucking would be impractical.
Trill: a pitch (usually a second above but may be any interval above or below) the written pitch is played, rapidly alternating between the written tone and the other.
Natural harmonic: the string is lightly touched above certain frets, plucked and then the left hand unstops the string if sustain is desired. The most common natural harmonics are at the 4th, 5th, 7th, 9th, and 12th frets. The sounding note is 2 octaves and a major third above, 2 octaves, a perfect 5th, 2 octaves and a major third, and an octave above the unstopped pitch respectively.
Artificial harmonic: instead of playing a harmonic with the string unstopped, the string is stopped and may be touched a specific number of frets above and played to produce a harmonic tone above the stopped note. Artificial harmonics can be done by stopping and touching with the left hand and plucking with the right or by stopping with the left hand, touching with the right and then plucking with another finger on the right hand.
Glissando: sliding up or down the fingerboard stopping all chromatic notes between the plucked note and the final note.
Fingered glissando: Instead of slurring as one slides up or down the fingerboard, each note is plucked essentially voicing all chromatic pitches between the starting and ending note.
Portamento: sliding up to a note before playing it. Gives a very expressive feel.
Staccato: the duration of a note is shortened and the note is articulated very quickly.
Pulsing: the left hand slightly presses and releases, pulsating the note.
Slide: Instead of stopping the strings with the left hand, a slide may be used. The slide is held on top of the fret, touching the strings lightly. When playing slide, it is common to use open tunings. Slides can be anything from glass and metal slides, to ratchets and coke bottles.
Source: Adler, Samuel. The Study of Orchestration 3rd ed. Norton & Company Inc. 2002