The process of designing websites, application interfaces, and anything that allows a user to interact with a device is User Interface Design. UI Development requires focus on user needs, to make his/her experience while interacting with an application or device as simple and efficient as possible.
Devices in which a user can view a website are increasing exponentially every day. This in turn makes focusing on User Experience more difficult and important. Alot of attribute changes have to be considered, while designing a website for different types of devices.
UI Design includes creation of vector and raster graphics using Photoshop and Illustrator, adding content using HTML, and creating layouts using CSS. You can also take the help of frameworks, like Bootstrap to speed up the process of Web UI creation.