Many straight-forward tutorials and lecture-notes can be obtained on the web by using basic searches, but this set of web-pages is foremost in presenting decision-tree learning to the newcomer in an appealing way. This is the only site to my knowledge along with this one that has some form of interactive decision tree material on it. The central forte of the site is that of the embedded Java applet that can be used to construct trees from several sample datasets with different node-splitting and pruning functions. This is done in real-time or can be done step-by-step with an algorithmic view allowing the user to examine the process in more detail. The use of the applet is interwoven with the narrative about a hypothetical basketball game involving the Mallrats. This one fact tells you that if the Mallrats can meaningfully be combined with decision tree learning, this site must be visited right away.
The Machine Learning Network
Information on a whole load of Machine Learning and Data Mining stuff. Some quite detailed information on decision tree learning can be found here (such as the AdaBoost algorithm).
CISpace Decision Tree Learning Applet
Nice interactive java applet for students. Also has other links to related applets and has some videos(!) you can watch on creating and testing decision trees!
ONLamp.com — Building Decision Trees in Python
Good article on using decision trees and coding them in Python
Information about the influential CART algorithm which also covers some general topics.
Pierre Guerts Decision Tree Applet
Another nice interactive applet.
Collection of Tutorials and videos on Data mining Tools.