Hashmap

• Not synchronized
• Accepts nulls and a null key
• You can sweep entire structures with a simple iterator
• It has containsValue() and containsKey()
• It's faster
• Consume less memory
• More modern
• Mother: AbstractMap

Hashtable

• Synchronized and can be easily used in concurrent environment
• Do not accept nulls
• Iterar is more complicated
• It has contains()
• Has overhead by synchronization
• Holds more memory
• Deprecated
• Mother Dictionary

I removed the information mainly from this answer in the SO .