Historically, most dictionaries were prescriptive. They explained the standard usage of the language as well as the proper spellings and grammatical rules. Throughout the 20th century, however, descriptive dictionaries began to become far more common.
Because of all of the outside forces acting on the English language -- the Internet, the influence of other languages and so on -- very few modern dictionaries are technically prescriptive. The last major American prescriptive dictionary was Noah Webster's "An American Dictionary of the English Language," published in 1828.
Major 20th- and 21st-century dictionaries like Merriam-Webster and Funk and Wagnall's attempt to describe the language as it is being used; therefore, they are technically descriptive dictionaries. Online, user-based dictionaries like the Urban Dictionary are also descriptive, but while these dictionaries can be useful, largely because they can be updated instantaneously, they are generally useful only for recreation purposes.