Two long-standing schools of thought have dominated discussion in grammar. The prescriptive school looks at the way the language ought to be used. Its adherents set out the rules of grammar as the standard to follow. The alternative, descriptive approach views language as living and evolving – language as it's used.
In The Glamour of Grammar: A Guide to the Mystery and Magic of Practical English, Roy Peter Clark thoroughly explores the shift from the notions of how people ought to speak (prescriptive) versus how they do in fact speak (descriptive).
For instance, “Where you at?” is a common question in current, regional discourse. The prescriptive approach would pronounce this a faulty use of grammar, arguing it violates both the rule of a complete sentence needing a verb, and the rule not to end a sentence with a preposition. Prescriptivists may go even so far as to suggest that the speaker is uneducated, using sub-standard English.
By contrast, the descriptive perspective would recognize this question as an expression commonly used. The only measure it must meet is: Does it, in fact, communicate? If the person hearing it understands what is being asked, it qualifies as acceptable, and may be considered even to be an advancement or evolution of the English language.