Web Designer - pretty much designs websites on the whole, including the user experience, information architecture and maybe a bit of front-end HTML & CSS

UI Designer - pretty much designs the interface only, and others worry about the UX, IA and coding, etc

Digital Designer - is more a blanket term for someone doing web design, multimedia, flash, video, motion graphics, etc

Web Designer is easier for others (especially those outside the industry) to understand what you do without an explanation