EUR-Lex is the official EU database for accessing EU law. It is available in 24 languages and includes treaties, legislation, international agreements, preparatory acts, case law and parliamentary questions. It also gives access to the Official Journal of the European Union.
However if you are just looking for someplace where EU is explained for a general audience in a concise and jargon-free language a good place to start is here.
Here is an example of one of the online tutorials. Why not have a look.