Define Implicit and Explicit Conversion in C#

Implicit conversion operators can be called implicitly, without being specified by explicit casts in the source code.

Unlike implicit conversion, explicit conversion operators must be invoked via a cast.

//Implicit conversion
int total = 100;
string str = "Total number is : ";
Console.WriteLine(str + total);

//Explicit Converion
Object obj = new object();
int i = (int)obj + 5;

http://msdn.microsoft.com/en-us/library/z5z9kes2(v=vs.71).aspx[^]
http://msdn.microsoft.com/en-us/library/xhbhezf4(v=vs.71).aspx[^]

Codeproject is always the best source, refer here,
Type conversions with implicit and explicit operators[^]