FontSlant is an option for Cell, StyleBox and StyleForm which specifies how slanted the characters should be in text in the cell.
Typical settings are "Plain", "Italic" and "Oblique".
With the "Oblique" setting, each character typically has the same basic form as with "Plain", but is slanted. With the "Italic" setting, the basic form is different.
See notes for FontFamily.
See The Mathematica Book: Section 2.8.9 and Section 2.10.10.
See also: AutoItalicWords, SingleLetterItalics.