At its simplest, an object holds a single value. At its most complex, it holds references to many other objects. The .NET Framework provides collections—these include List and Dictionary. They are often useful.

List

First, the List type provides an efficient and dynamically-allocated array. It does not provide fast lookup in the general case—the Dictionary is better for this. List is excellent when used in loops.
List

Program that uses List type [C#]

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
 // Use the List type.
 List<string> list = new List<string>();
 list.Add("cat");
 list.Add("dog");

 foreach (string element in list)
 {
     Console.WriteLine(element);
 }
    }
}

Output

cat
dog

Dictionary

The Dictionary type in the base class library is an important one. It is an implementation of a hashtable, which is an extremely efficient way to store keys for lookup. The Dictionary in .NET is well-designed.
Dictionary

Program that uses Dictionary [C#]

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
 // Use the dictionary.
 Dictionary<string, int> dict = new Dictionary<string, int>();
 dict.Add("cat", 1);
 dict.Add("dog", 4);

 Console.WriteLine(dict["cat"]);
 Console.WriteLine(dict["dog"]);
    }
}

Output

1
4

Generic Class

ArrayList

Next, the ArrayList is a collection found in System.Collections and it can store objects of any derived type. You don't need to worry about the type of the elements, at least until you need to use them.
ArrayList

Program that uses System.Collections [C#]

using System;
using System.Collections;

class Program
{
    static void Main()
    {
 ArrayList list = new ArrayList();
 list.Add("cat");
 list.Add(2);
 list.Add(false);

 foreach (var element in list)
 {
     Console.WriteLine(element);
 }
    }
}

Output

cat
2
False

Hashtable

To continue, the Hashtable is a lookup data structure that uses a hash code to find elements quickly. The newer Dictionary collection is usually more appropriate for programs when available.
Hashtable

Program that uses Hashtable [C#]

using System;
using System.Collections;

class Program
{
    static void Main()
    {
 Hashtable table = new Hashtable();
 table["one"] = 1;
 table["two"] = 2;

 // ... Print value at key.
 Console.WriteLine(table["one"]);
    }
}

Output

1

Lists

Lists are linear—one element is stored after the other. The List generic type is often the best implementation available for its purpose on the .NET Framework. There are other versions of lists.
LinkedList SortedList ReadOnlyCollection Capacity

Tables

There are many versions of lookup data structures other than the Hashtable and Dictionary in the .NET Framework. Usually, a Dictionary is the best option—but sometimes custom features are needed.