A: First of all, depending upon the site of refactoring one can distinguish several types of it: code refactoring, database (schema) refactoring, refactoring of unit tests, refactoring of GUI etc.

There are several situations where you can meet refactoring during software development:

1. Refactoring is known to be a mandatory step in certain agile development techniques like test-driven development. It is supposed to perform refactoring step after every implementation step. In this case the refactoring targets just the last implementation and its goal is to integrate the new code into the existing code corpus in the most optimal way.

2. Refactoring can be done some internal problems in the working code are detected: this is called "code smell". This estimation is in many aspects rather subjective, despite the fact that it can be actually based upon certain code metrics (like number of lines of code per method, cyclomatic complexity of the code etc.). Here the goal of refactoring is to improve the code quality by changing it so that the metrics used for quality estimation return to the expected domain.

3. You often need to refactor the code to achieve certain principles of programming in your code, look for Clean Code development to learn more about such principles.

4. You may need to perform refactoring of your code and database schema to prepare it for coming changes, especially if those were not considered during the design phase of the project. For example data normalization and denormalization take often place during data-driven software development to prepare the database for possible extensions.

Refactoring tools available on the market basically support the developer in two ways:

1. While writing your code, you get suggestions how you can improve it "on-the-fly". Whereas many fallacies can be detected directly by your IDE, like Visual Studio or Eclipse (for example dead code, variables declared but not used etc.), the refactoring tools like Resharper can reveal problems which are far less evident, like re-writing the loops in LINQ queries etc.

2. These tools also support you with custom refactoring steps, like global renaming of your identifiers, splitting your class declarations into separate properly named files, extracting interfaces and base classes from your class implementation etc. They save a lot of work here, especially if your project has a large code base, but you must first know what you really want to refactor.

Actually using tools like ReSharper in everyday's development is so useful that it makes you almost dependent on them: they really accelerate the process of code writing, especially if you know how to use them appropriately!