Most programmers represent floating-point numbers with type double. In fact, C# treats all real numbers you type in an application’s source code (such as 7.33 and 0.0975) as double values by default. Such values in the source code are known as floating-point literals. To type a decimal literal, you must type the letter "M" or "m" at the end of a real number (for example, 7.33M is a decimal literal rather than a double). Integer literals are implicitly converted into type float, double or decimal when they are assigned to a variable of one of these types.
Reference: Visual C#® 2005: How to Program, Second Edition.