Arraylist itself is a part of collection. 
From a .NET perspective, a collection could be defined as 
an object that implements one or more of the 
System.Collections.ICollection, 
System.Collections.IDictionary, and 
System.Collections.IList interfaces.
 
This definition leads to my classification of the "built-
in" collections found in the System.Collections namespace 
into three broad categories:
Ordered collections: Collections that implement only the 
ICollection interface are usually distinguished by the fact 
that insertion order controls the order in which objects 
may be retrieved from the collection. The 
System.Collections.Stack and System.Collections.Queue 
classes are both examples of ICollection collections.
Indexed collections: IList-implementing collections are 
distinguished by the fact that their contents can be 
retrieved via a zero-based numeric index, like an array. 
The System.Collections.ArrayList object is one example of 
an indexed collection.
Keyed collections: Collections implementing the IDictionary 
interface contain items that can be retrieved by an 
associated key value of some kind. The contents of 
IDictionary collections are also usually sorted in some 
fashion based on the key valu and can be retrieved in 
sorted order by enumeration. The 
System.Collections.HashTable class implements the 
IDictionary interface.

ArrayList
 
The System.Collections.ArrayList class, which implements 
only IList, could best be described as a hybrid of a normal 
array and a collection. ArrayLists hold items in the order 
they were added. Items are assigned an index identifier and 
can be retrieved in any order via their associated index 
number. The ArrayList grows as new items are added to it, 
which makes it more flexible than a normal array. However, 
an ArrayList has more overhead than a traditional array and 
is not strongly typed, accepting anything that can be cast 
to System.Object (in other words, everything).

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