Q:

In Python, what is the difference between an object and a dictionary?

After an object has been created, I can add and remove slots at will, as I can do with a dictionary. Even methods are just objects stored in slots, so I probably can add methods to a dictionary as well.
Is there something I can do with a (non-dictionary) object that I could never do with a dictionary?
Or is it possible to set up a dictionary that completely looks like an object of a certain class?
This question is not about how they are created, but about how I can use them afterwards. Links or references are appreciated.

A:

After an object has been created, I can add and remove slots at will, as I can do with a dictionary. Even methods are just objects stored in slots, 

Be careful saying slots -- __slots__ has a specific meaning in Python.

so I probably can add methods to a dictionary as well.

foo = {}
foo.bar = 1

AttributeError: 'dict' object has no attribute 'bar'

Not by default, you'd need to subclass it -- but then you're using a class. Instead, put the function in the dictionary:
def bar():
    return 1

foo['bar'] = bar
foo['baz'] = lambda: 1

Is there something I can do with an object that I could never do with a dictionary?

This question actually has a false premise -- dictionaries are objects, so anything you do with a dictionary you are doing with an object. For the rest of this answer, I'll pretend you mean "user defined class" when you say "object".
No, there is nothing you can do with a user-defined class you can't do with a dictionary. Classes are even implemented with a dictionary.
class Foo(object):
    pass

print Foo.__dict__
# {'__dict__': <attribute '__dict__' of 'Foo' objects>, '__module__': '__main__', 
#      '__weakref__': <attribute '__weakref__' of 'Foo' objects>, '__doc__': None}

Anything you can do with a class in Python you can do without one, with more work (and code which is much harder to understand, use, and maintain). It's even been said that Python classes are just syntactic sugar for dictionaries.
A very similar question was asked recently as Do I need to learn about objects, or can I save time and just learn dictionaries? I think my answer to that is also relevant here.

A:

When you call member functions of an object, they get passed the object itself as a first parameter. This way they can modify properties of the object they belong to:
class Counter(object):
   def __init__(self):
      self.num = 0

   def inc(self):
      self.num += 1

count = Counter()
count.inc()

If you just store a bunch of functions in a dict, they will get no such support. You have to pass the dict around manually to achieve the same effect:
def Counter_inc(d):
   d['num'] += 1

def Counter_create():
   d = {
      'num': 0,
      'inc': Counter_inc,
   }
   return d

count = Counter_create()
count['inc'](count)

As you can see while it is possible, it is much more clumsy. While you can emulate many features of object with raw dicts, having special language support for objects makes them easier to use.
Also there are features that directly use an objects type information, like for example isinstance(), which cannot easily be replicated with raw dicts.

A:

The other answers hold some ground, but I'd like to add that you can't subclass or instantiate specific dictionaries.
class A(object): pass
class B(A): pass
a = A()

A = {}
B = ???
a = ???

So, dictionaries are objects and objects are implemented mainly with dictionaries (although I don't know the specifics), but they are different tools for different jobs.

