Basically, you can freely organize your thoughts on a project, a book, your To Do list and other type of information.
Starting with MindScape on iOS, users are able to add a topic then tap on it to pull up an empty page to fill out more details on that topic or add any number of subtopics. Resizing and moving around the topics is very easy.
For applications that support multiple platforms try one of the following:
XMind or Mind Meister. These will give you demos to let you see if Mind Mapping software is beneficial to you.
If not, there's always a lot of ToDo list programs from simple to powerful like
Remember the Milk Wunderlist Todoist
ToodleDo Google Tasks