I think this is a fairly common and challenging topic for anyone that has ever inherited code from another programmer or team. I can think of two common scenarios in my professional career that I've run into; diving into an open source project, and strictly inheriting the code of an individual's project.
I'm coming up on the latter and will be inheriting a project created by an individual, and while thinking about how to best approach this, I figured I would ask for the intelligence of p.se so I can learn from your experiences.
When Magento (large scale eCommerce application) came out, I was really lost in the Zend framework. There was little documentation out during alpha, so it was hard to use the community resources. I ended up using
grep a lot and back tracing methods to get a firm understanding of what it does. By doing that, I eventually learned more about the application design, although it did take a little while.
What is your approach when you're up against a project that has little documentation? I feel like it is almost finishing the painting of someone else, which is always an interesting task.