A clef (Lat. clavis: key) is a symbol written at the beginning of each line of music which determine the pitch-range of the staff.
A clef may also be written at any point a new pitch-range is to begin.
The treble clef (also G clef) looks like a stylised 'G' and places the pitch G4 on the second line from the bottom. It is used for high-pitched instruments, high-voices, and right-hand piano parts.
Two great mnemonics for remembering the treble clef are: Every Good Boy Deserves Fruit and simply FACE.
Figure 1: A treble clef with pitches written.
The bass clef (also F clef) looks like a stylised 'F' and places the pitch F3 on the second line from the top. It is used for low-pitched instruments, bass voice, left-hand piano parts, and bass guitar.
Two great mnemonics from remembering the bass clef are: Good Boys Deserve Fruit Always and All Cows Eat Grass.
Figure 2: A bass clef with pitches written.
The alto clef belongs to the C clef family and places middle C (C4) on the middle line. Formally used for alto voice, it is now mainly used for viola notation.
Figure 3: An alto clef with middle C.
The tenor clef belongs to the C clef family and places middle C (C4) on the second line from the top. It is almost obsolete, but still sometimes used for cello, tenor, trombone, and bassoon notation.
Figure 4: A tenor clef with middle C.
The soprano clef belongs to the C clef family and places middle C (C4) on the first line from the bottom. It is now an obsolete clef formally used in mediaeval music and some works well into the 19th century.
Figure 5: A soprano clef with middle C.
The neutral clef is used for percussion notation. Each line or space represents a percussion instrument rather than a pitch.
Figure 6: A neutral clef.
Tablature is a form of musical notation which is written as fingering positions rather than notes. It is highly adopted by music for guitar, ukulele, lute, and other stringed instruments.
The number of lines in a tablature staff is not always five, but rather the number of strings or voices used for a particular instrument.
Figure 7: Tablature clef for a six-string guitar.
Also see Staff | Bar.