System.Collections.Generic.IDictionary<TKey, TValue>

Added in .NET 2.0
This is the interface for a key/value pair collection.

It supports all the methods of ICollection<T> and IEnumerable<T>
Does the dictionary contain a particular key
Does the dictionary contain a particular key and value
Add entry
Remove entry
Get the value corresponding to a given key (indexing operator)
Returns all the keys
Returns all the values

System.Collections.Hashtable

There is no non-generic class called Dictionary.
The non-generic equivalent of Dictionary<TKey, TValue> is called Hashtable.
When anyone refers to a Dictionary they are referring to the generic class.