If terms such as SQL injections and cross-site scripting attacks mean nothing to you, you'll probably want to stop reading round about now. But if you're familiar with the terms, and would like to know more about the topic, then you'll be interested in some training materials that have been published for free online by OWASP, the Open Web Access Security Project. This group is dedicated to providing resources to help making based systems more secure, and the PowerPoint slides from a bunch of its recent training courses have been made available.
They're freely editable and downloadable. Just head to https://www.owasp.org/index.php/Education/Free_Training and click on any of the links in the Training Materials column.
Note that the slides don't include much narrative, so you'll need to be relatively aware of the subject to make the most efficient use of these presentations.