Where – when asking questions about location.
What is the difference?
Where are you?
Where are you at?

So, what is the difference? Basically, we use the “at” version in colloquial English. For example, in online multiplayer games. As a matter of fact, The gamers I play with, often use the at version.
Also on question words:
Introduction to tenses
- Components: Verb, Action Verb, Stative Verb
- Verb: Where is the border between Action and Stative Verbs?
- Action vs Inaction
- Frequency of actions (/inactions and states)
- Time of actions (signal words)
- 5 Types of Sentences
- Declarative Sentence
- Imperative Sentence
- Interrogative Sentence
- Exclamatory Sentence
- Present
- Future
- Past
- Moment of speaking
- Finished vs unfinished vs unspecified period of time
- Continuous Actions in Linguistics
Links on Present Simple
- Present Simple: Why Simple? Isn’t it complicated?
- Present Indefinite why indefinite?
- Repeated actions: Present Simple tense
- Frequency words: Present Simple with repeated actions
- General realities and truths, world truths: Present Simple
- Stating facts in Present Simple Tense
- Unchanging (permanent) situations: Present Simple
- Present Simple: Past actions! Jokes, stories, books, etc.
- Present Simple: Future meaning for fixed actions
- Actions / states ongoing now: Present Simple