When I want to add some features to my Craft site, do I need to modify the files in the craft/app folder? I am new to Craft CMS, now I run a demo site of Happy Lager, wanna add a web crawler to fetch some data to my db. I should develop a plugin to complete this feature, but if I need to change the craft/app folder? or how my services in plugin called by my site? call from the craft/app? please help me,thank you!
Short answer... no!!! You don't ever want to make changes to the craft/app folder. Any changes you make in the app folder will be overwritten when you update Craft.

If you need any custom behavior, the correct way to do it is to write a custom plugin. Your plugin will be safe outside of the Craft core, and will survive any Craft updates intact.

Read up on the complete documentation for creating a Craft plugin. You can also download the Business Logic template to see what some basic components look like. Or even better, spin up a custom plugin using the fantastic Plugin Factory!