A dictionary is certainly helpful when you don't know something. It could be a word or a term.
For an English word, I generally go to dictionary.com. It has detailed definitions for any English word and it can pronounce for you.
The downsides are dictionary.com has too many advertisements. Also very often it can only pronounce one particular part of speech of a word. For example it can pronounce 'physical' but not 'physically'. How weird is that?
I am sure there are many online dictionaries out there for every language humanly known. Remember: Google is your friend =)