Learning the topology needed for topos theory. I have just started learning topos theory and I am going through Mac Lane and Moerdijk's book, "Sheaves in Geometry and Logic". I have, unfortunately, very little experience with topology. I started reading their second chapter, "Sheaves of Sets" but I can not follow. Is there a text, maybe, that someone can recommend, that introduces the basics of topology, from a categorical view point? I do not want a career in topology, I only want to learn what is needed in order to be able to follow material like what is included in the chapter mentioned above. Thanks for any help!
 A: Hi AlistairDermont and welcome to Mathematics.SE.
If you wish to learn a bit of general topology and then use it for whatever legal purpose it may serve, as they say, I would suggest that you look at Schaum's General Topology by S. Lipschutz. It is perfect for self-learning with its wealth of solved exercises.
Chap. 5 starts with the real juicy stuff.
In parallel I also suggest you look at Wikipedia's related pages and references therein (Munkres and Kelley are standard). 
This page will give you an overview of what's available:
http://en.wikipedia.org/wiki/List_of_general_topology_topics
If you wish to study basic category theory without general topology then the best option is Awodey's book whose first edition can also be found online.
A: In order to learn category/topos theory, it is essential that you get comfortable with topology. For example, in order to get the intution for Grothendieck topologies, you should be familiar with topology.
This turns the question more into a reference request for topology textbooks. Let me recommend a few books which I personally appreciated. I liked Topology without Tears, S. Morris. Munkres is a great book; in order to gain intuition for Munkres, do the exercises. General Topology by Stephen Willard and Basic Topology by M.A. Armstrong are also pretty good books. I wouldn't recommend this for studying, but Singer and Thorpe, Lecture Notes on Elementary Topology and Geometry is a great book. It does not have any exercises, and so is not a textbook like Munkres. (You could check out Allen Hatcher's Notes on Introductory Point-Set Topology is you wish to.)
Lastly, check out Allen Hatcher's recommendations.
A: The book by Mendelson (Introduction to topology) is quite nice.
Near the end it has a very introductory discussion of the fundamental group.
As well as the basic language of topological spaces, a very useful thing
to learn would be the theory of covering spaces (which builds on the theory
of the fundamental group, at least the way it is usually presented).
The point is that a basic example of a sheaf of sets is the sheaf of sections
of a covering space (and it's pretty hard to appreciate the concept of sheaf
without being able to understand this example).
There are many texts which explain covering spaces, of course; the first chapter
of Hatcher's book on algebraic topology is one such place.  (Note that you don't
need to read on and learn the material of the later chapters, although
of course that material is also very interesting; I am specifically recommending
covering space theory here.  Unfortunately most general topology texts
don't cover this theory.)
