In the past few years, Sketch has become a de facto industry standard amongst digital designers. It is a more lightweight application than tools like Photoshop and Illustrator, and often allows designs for websites and apps to be put together more quickly. Sketch is currently only available for Mac.
Even more recently, Figma has emerged as an excellent, and free, browser-based alternative to Sketch. It is great for design teams and other forms of collaborative design work, and can be used on PCs and Chromebooks. We have a free email course about Figma as well!