DICTIONARIES
The web is full of dictionaries. This page shows links only to a small selection of them. 

Making use of dictionaries  when reading and writing in English can hardly be escaped. However, using web and ordinary dictionaries can be difficult because of the various meanings and usages of words. If you find using the dictionaries listed here a problem, click tasks to have more practice.

MONOLINGUAL DICTIONARIES (ENGLISH-ENGLISH)
The largest dictionary available is the Oxford English Dictionary. It gives you a lot of information about the etymology ("history") of the words in it. Experiment both with the "search" and "find word" options.
Another useful large dictionary is Merriam-Webster's dictionary.
The Wordsmyth English dictionary  is relatively easy to use.
This link will take you to a web site that offers links to subject-specific dictionaries.
BILINGUAL DICTIONARIES
Through this link you can connect to the Kielikone dictionary which is good choice if you prefer using bilingual dictionaries.