Builder is an object creational design pattern that codifies the construction process outside of the actual steps that carries out the construction - thus allowing the construction process itself to be reused.  

Non-software Example 

Fast food restaurants to construct children's meals use this pattern. Children's meals typically consist of a main item, a side item, a drink, and a toy (e.g., a hamburger, fries, Coke, and toy car). Note that there can be variation in the content of the children's meal, but the construction process is the same. Whether a customer orders a hamburger, cheeseburger, or chicken, the process is the same. The employee at the counter directs the crew to assemble a main item, side item, and toy. These items are then placed in a bag. The drink is placed in a cup and remains outside of the bag. This same process is used at competing restaurants. [Michael Duell, "Non-software examples of software design patterns", Object Magazine, Jul 97, p54] 

Another example for Builder pattern is a Computer Assembly. A computer is nothing but the bundling of various components like FDD, HDD, Monitor etc. But when an user buys a computer someone assemble all these components and given to us. Remember that here the building process is completely hidden from the client or user. 

The UML diagram for a Builder pattern is more or less like following one.  

Remember that a project can contain one or more builders and each builder is independent of others. This will improves the modularity and makes the addition of other builders relatively simple. Since each builder constructs the final product step by step, we have more control over the final product that a builder constructs.

C# implementation

//Creational Pattern: BUILDER
using System;
class Director
{
public void Construct(IBuilder builder)
{
builder.DoIt();
}
}
interface IBuilder
{
void DoIt();
}
class BuilderA : IBuilder
{
public void DoIt()
{
//Necessary code for building the computer type A
Console.WriteLine("Assembly a Computer with mono monitor");
}
}
class BuilderB : IBuilder
{
public void DoIt()
{
//Necessary code for building the computer type B
Console.WriteLine("Assembly a Computer with color monitor");
}
}
class MyClient
{
public static void Main()
{
Director d = new Director();
IBuilder build = new BuilderA();
d.Construct(build);
}
}