Did you find a thick Webster's dictionary, printed on newsprint, for $2.99, in the supermarket?  A bargain?  

Probably not.  Webster is not a copyrighted name.  Anybody can use it--and many do. If you want the Webster's, look for Merriam-Webster's dictionary.

prescriptive and descriptive 
Merriam-Webster's is a descriptive dictionary; that is, selection of words is based upon the words actually being used by people.

If you want your dictionary to guide you in the "worthiness" of a word, you may want to choose a different dictionary.  (Many linguists are dismissive about prescriptive approaches to language and grammar.)

Merriam-Webster's is nevertheless a fine dictionary.  Another you may want to consider is the American Heritage® Dictionary of the English Language.

old dictionary.  What's the matter with the dictionary your mother used in college?  Nothing--except it probably doesn't have lots of words in it that you want to know.  Language changes, and nowadays English seems to be changing at a breakneck pace.