**Roy Joshua**

Ohio State University, U.S.A.

August 23, 2011

**What is an Algebraic stack?**:
This will be an overview of stacks and algebraic stacks, assuming
nothing more than familiarity with some elementary notions in algebraic geometry (as discussed, for example, in undergraduate or first year graduate courses).
Algebraic stacks have become important in recent years due to their
applications in various fields ranging from mathematical physics to the geometric Langlands' program. In this talk we will first try to motivate the use of stacks, by considering a few basic examples. After some basic definitions, we will review how to define various cohomology-homology theories for algebraic stacks and conclude with some notable results on algebraic stacks.