St.

St. is an abbreviation for

*Street
*Saint (although in British English the trailing period is commonly dropped, that is, as "St" rather than "St.")

The usage is understood by context. For example, the expression "St. Peter St." would be assumed to mean "Saint Peter Street".

Ste. is the abbreviation for the French "Sainte", the feminine form of "Saint", commonly seen in English references, e.g. Sault Ste. Marie.