This example generates random values between 0 and 2147483647.

Random rnd = new Random();
int randomNumber = rnd.Next();

Generate a random number between 0 and 1.0. (not including 1.0)

Random rnd = new Random();
var randomDouble = rnd.NextDouble();

Generate a random number between minValue and maxValue - 1.

Random rnd = new Random();
var randomBetween10And20 = rnd.Next(10, 20);

When creating Random instances with the same seed, the same numbers will be generated.

int seed = 5;
for (int i = 0; i < 2; i++)
{
   Console.WriteLine("Random instance " + i);
   Random rnd = new Random(seed);
   for (int j = 0; j < 5; j++)
   {
      Console.Write(rnd.Next());
      Console.Write(" ");
   }

   Console.WriteLine();
}

Output:

Random instance 0
726643700 610783965 564707973 1342984399 995276750
Random instance 1
726643700 610783965 564707973 1342984399 995276750

Two Random class created at the same time will have the same seed value.

Using System.Guid.NewGuid().GetHashCode() can get a different seed even in the same time.

Random rnd1 = new Random();
Random rnd2 = new Random();
Console.WriteLine("First 5 random number in rnd1");
for (int i = 0; i < 5; i++)
    Console.WriteLine(rnd1.Next());

Console.WriteLine("First 5 random number in rnd2");
for (int i = 0; i < 5; i++)
    Console.WriteLine(rnd2.Next());

rnd1 = new Random(Guid.NewGuid().GetHashCode());
rnd2 = new Random(Guid.NewGuid().GetHashCode());
Console.WriteLine("First 5 random number in rnd1 using Guid");
for (int i = 0; i < 5; i++)
    Console.WriteLine(rnd1.Next());
Console.WriteLine("First 5 random number in rnd2 using Guid");
for (int i = 0; i < 5; i++)
    Console.WriteLine(rnd2.Next());

Another way to achieve different seeds is to use another Random instance to retrieve the seed values.

Random rndSeeds = new Random();
Random rnd1 = new Random(rndSeeds.Next());
Random rnd2 = new Random(rndSeeds.Next());

This also makes it possible to control the result of all the Random instances by setting only the seed value for the rndSeeds. All the other instances will be deterministically derived from that single seed value.

Generate a random letter between a and z by using the Next() overload for a given range of numbers, then converting the resulting int to a char

Random rnd = new Random();
char randomChar = (char)rnd.Next('a','z'); 
//'a' and 'z' are interpreted as ints for parameters for Next()

A common need for random numbers it to generate a number that is X% of some max value. this can be done by treating the result of NextDouble() as a percentage:

var rnd = new Random();
var maxValue = 5000;
var percentage = rnd.NextDouble();
var result = maxValue * percentage; 
//suppose NextDouble() returns .65, result will hold 65% of 5000: 3250.