Mssql_result() returns the contents of one cell from a MS SQL result set. The field argument can be the field's offset, the field's name or the field's table dot field's name (tablename.fieldname). If the column name has been aliased ('select foo as bar from...'), it uses the alias instead of the column name.
When working on large result sets, you should consider using one of the functions that fetch an entire row (specified below). As these functions return the contents of multiple cells in one function call, they're MUCH quicker than mssql_result(). Also, note that specifying a numeric offset for the field argument is much quicker than specifying a fieldname or tablename.fieldname argument.