The most important thing that a developer needs to be aware of is modular development in magento 2. In magento 2 there is nothing like code pools, it’s completely removed.
In magento 2 everything resides under different modules. For example creating a new functionality to customising an existing functionality, you need to create a module.
For creating modules in magento 2, first we need to learn basics of magento 2 and folder structure and architecture of it.
After magento 2 installation, we need to create code directory inside app directory in application root. Inside app/code directory all custom or third-party modules resides.
For theme or design there is app/design directory. You can override default theme layout for adminhtml and frontend both sections by creating a new theme inside app/design/adminhtml/Magento and app/design/frontend/Magento directories.