StylePrint[expr, "style"] creates a new cell in the evaluation notebook with the specified style, and prints expr into it.
StylePrint[expr] uses the default style for the current notebook.
StylePrint creates a new cell after the input cell that is currently being evaluated.
StylePrint[expr, "style", opts] can be used to specify options for the cell that is created.
StylePrint is a special case of NotebookWrite.
With a text-based front end, StylePrint does the same as Print.
Cells generated by StylePrint by default have GeneratedCell->True and CellAutoOverwrite->True.
StylePrint[expr, "style"] generates a whole cell with the specified style; Print[StyleForm[expr, "style"]] generates a cell with the default style but containing a StyleBox object.
See The Mathematica Book: Section 2.8.19 and Section 2.10.1.
See also: CellPrint, Print, NotebookWrite, NotebookPrint, StyleForm.