A charter is a brief statement of what should be tested, how it should be tested, and problems that should be looked for, with the objective of having a clear mission for the session. The skill here, is to create a good charter quickly that both guides and plans the testing. The skill involves: thinking, planning, creativity, tests design, and likely writing.
A context where I used this skill:
I was working in a testing group of 10 people on a pricing science web application. Historically, at this company, testing was a highly scripted activity. Testers, developers, and product managers would plan a sprint and do design work, and after that, developers would go off and develop and testers would go off and start writing test cases. After a couple years of existence, maintenance of the previously documented tests became a non-‐‑trivial activity to the extent that sprint time had to be allocated to make old no longer valid tests relevant again. The group I was working with also noticed that most issues found were not from running the documented test again. Chartering was effective for us in the sense that we were able to spend more time testing software and that the documentation came to represent the testing that actually happened. --Justin Rohrman
I find the idea of test chartering to be closely related to defining test objectives. Where determining test objectives is a fairly high level concept, the test charter addresses how we intend to test these points of interest. The challenge is to get people to understand why something is important. I use pairing a great deal in determining charters to help reduce bias in perspective. -- Pete Walen
How I'd recommend someone learn this skill:
Start by reading: Chapter 2 of Explore IT by Elisabeth Hendrickson is devoted to the topic of chartering as well as its application in exploratory testing. Slide 22 of the Exploratory Testing PDF from Michael Bolton provides a clear definition.
Next creating charters is a learned-‐‑practice skill. By this, we mean the tester must write a charter, may be get it reviewed, and then use the charter in test. Next: wax on, wax off, over and over. Also, it can be very useful to do pair testing with someone who is good at doing a charter. Do the charters and testing rapidly. And do not be afraid to make mistakes, but when you do make a mistake, learn from it.
Note: on the net, there are many example charter formats and contents. These are only starting point examples to learn from. You should create (think) your own charter.