Simply, Dictionary<TKey,TValue> is a generic type, allowing:
- static typing (and compile-time verification)
- use without boxing
If you are .NET 2.0 or above, you should prefer Dictionary<TKey,TValue> (and the other generic collections)
A subtle but important difference is that Hashtable supports multiple reader threads with a single writer thread, while Dictionary offers no thread safety. If you need thread safety with a generic dictionary, you must implement your own synchronization or (in .NET 4.0) use ConcurrentDictionary<TKey, TValue>.