When you declare a variable in C# it will be saved as either a Value or Reference Type.
These two types refer to how the variable is stored in memory on the computer.

Value Types

Value types store the value within it's own memory space, that is to say that the memory location that the variable is pointing to contains the actual value.

Let's take a look at an example.
We'll declare an integer i with a value of 100. The computer will then store the value 100 in the memory location that the variable i is pointing to:

Diagram showing how value types are stored in computer memory.

Reference Types

Reference types store their values in a different way to value types. Reference types store the address of where the value is being stored.

Let's demonstrate with another example.
Declare a string s = "Hi". The computer will store s in a location; 0x247439 in our case. The value of s is 0x247442 which is just the address of the actual data value.

s is just a reference to where the actual data is stored.

Diagram showing how reference types are stored in computer memory in the C# Language

Simple Reference Type Test

Here's a simple test that shows that two variables can reference the same object:

public void TwoVarsCanReferenceSameObject()
  // arrange
  var book1 = GetBook("Book 1");
  var book2 = book1;

  // assert
  Assert.Same(book1, book2);
  Assert.True(Object.ReferenceEquals(book1, book2));

InMemoryBook GetBook(string name)
  return new InMemoryBook(name);

Built-In Types for the C# Language

C# type keyword .NET type
bool System.Boolean
byte System.Byte
sbyte System.SByte
char System.Char
decimal System.Decimal
double System.Double
float System.Single
int System.Int32
uint System.UInt32
nint System.IntPtr
nuint System.UIntPtr
long System.Int6
ulong System.UInt64
short System.Int16
ushort System.UInt16

The following table lists the C# built-in reference types:

C# type keyword .NET type
object System.Object
string system.String
dynamic System.Object

The preceding two tables were taken from https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/built-in-types

Special Case:

String is defined by a class in C#. It is always a Reference type, but it behaves like a value type.

Strings are immutable. This is a key point!

Chat soon.