While there are consistent systems of applying chord symbols, different systems are used and may be used by the same person.
Roman numerals may be used to indicate the scale degree upon which the chord is built. Letter notation may be used to indicate the note upon which the chord is built, some people use a small and large letter to indicate minor and major, respectively. Various abbreviations are used to indicate the quality of the chord.
For a I chord in C major, the various systems look like thus
{| params border=1 |Quality |Roman Numeral |Letter (C) |Short abb. |Long abb. |symbol |- |Major |I |C |M |Maj | |- |Augmented |I |C |A |Aug |"+" |- |minor |i |c or C |m |min |"-" (real book) |- |diminished |i |c or C |d |dim |° |}
The chord symbols for seventh chords can be found at that article.
See: inversion (music), and figured bassInversions and figured bass