What is the difference between Lists, ArrayLists, Maps, Hashmaps, Collections etc..?
The API is pretty clear about the differences and/or relations between them:
Collection
The root interface in the collection hierarchy. A collection represents a group of objects, known as its elements. Some collections allow duplicate elements and others do not. Some are ordered and others unordered.
http://download.oracle.com/javase/6/docs/api/java/util/Collection.html
List
An ordered collection (also known as a sequence). The user of this interface has precise control over where in the list each element is inserted. The user can access elements by their integer index (position in the list), and search for elements in the list.
http://download.oracle.com/javase/6/docs/api/java/util/List.html
Set
A collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2), and at most one null element. As implied by its name, this interface models the mathematical set abstraction.
http://download.oracle.com/javase/6/docs/api/java/util/Set.html
Map
An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.
http://download.oracle.com/javase/6/docs/api/java/util/Map.html
Is there anything in particular you find confusing about the above? If so, please edit your original question. Thanks.
A short summary of common java collections:
'Map': A 'Map' is a container that allows to store key=>value pair. This enables fast searches using the key to get to its associated value. There are two implementations of this in the java.util package, 'HashMap' and 'TreeMap'. The former is implemented as a hastable, while the latter is implemented as a balanced binary search tree (thus also having the property of having the keys sorted).
'Set': A 'Set' is a container that holds only unique elements. Inserting the same value multiple times will still result in the 'Set' only holding one instance of it. It also provides fast operations to search, remove, add, merge and compute the intersection of two sets. Like 'Map' it has two implementations, 'HashSet' and 'TreeSet'.
'List': The 'List' interface is implemented by the 'Vector', 'ArrayList' and 'LinkedList' classes. A 'List' is basically a collection of elements that preserve their relative order. You can add/remove elements to it and access individual elements at any given position. Unlike a 'Map', 'List' items are indexed by an int that is their position is the 'List' (the first element being at position 0 and the last at 'List.size()'-1). 'Vector' and 'ArrayList' are implemented using an array while 'LinkedList', as the name implies, uses a linked list. One thing to note is, unlike php's associative arrays (which are more like a Map), an array in Java and many other languages actually represents a contiguous block of memory. The elements in an array are basically laid out side by side on adjacent "slots" so to speak. This gives very fast lookup and write times, much faster than associative arrays which are implemented using more complex data structures. But they can't be indexed by anything other than the numeric positions within the array, unlike associative arrays.
To get a really good idea of what each collection is good for and their performance characteristics I would recommend getting a good idea about data structures like arrays, linked lists, binary search trees, hashtables, as well as stacks and queues. There is really no substitute to learning this if you want to be an effective programmer in any language.
You can also read the Java Collections trail to get you started.
In Brief (and only looking at interfaces):
List - a list of values, something like a "resizable array"
Set - a container that does not allow duplicates
Map - a collection of key/value pairs