Usually we are using String.Format method for formatting purpose, the.ToString is usually used for converting other types to string. We can specify the format along with the ToString method while conversion is taking place, So we can avoid an additional Formatting. Let Me Explain how it works with different types;

Integer to formatted string:

int intValue = 10;
string zeroPaddedInteger = intValue.ToString("000"); // Output will be "010"
string customFormat = intValue.ToString("Input value is 0"); // output will be   "Input value is 10"

double to formatted string:

double doubleValue = 10.456;
string roundedDouble = doubleValue.ToString("0.00"); // output 10.46
string integerPart = doubleValue.ToString("00");    // output 10
string customFormat = doubleValue.ToString("Input value is 0.0");  // Input value is 10.5

Formatting DateTime using ToString

DateTime currentDate = DateTime.Now; //  {7/21/2016 7:23:15 PM}
string dateTimeString = currentDate.ToString("dd-MM-yyyy HH:mm:ss"); // "21-07-2016 19:23:15"
string dateOnlyString = currentDate.ToString("dd-MM-yyyy"); // "21-07-2016"
string dateWithMonthInWords = currentDate.ToString("dd-MMMM-yyyy HH:mm:ss"); // "21-July-2016 19:23:15"